Update docs:

- Added audio troubleshooting section
- Added profile section to playback
- formatting Details section of clips

doc/clips.rst

@@ -204,12 +204,9 @@ position, to help create animations.
    Waveform                Bool        Should a waveform be used instead of the clip's image
    ======================  ==========  ============
 
-Details
+Gravity
 """""""
-
-**Gravity**:
-
-  Gravity sets an initial position for the clip, once it has been scaled as above. The options are:
+Gravity sets an initial position for the clip, once it has been scaled as above. The options are:
 
   - *Top Left* – the top and left edges of the clip align with the top and left edges of the screen
   - *Top Center* – the top edge of the clip aligns with the top edge of the screen; the clip is horizontally centered on the screen.
@@ -221,24 +218,26 @@ Details
   - *Bottom Center* – the bottom edge of the clip aligns with the bottom edge of the screen; the clip is horizontally centered on the screen.
   - Bottom Right – the bottom and right edges of the clip align with the bottom and right edges of the screen
 
-**Scale**:
-
- This is the initial resizing method, which may be further adjusted by Scale X and Scale Y (below). The options are:
+Scale
+"""""
+This is the initial resizing method, which may be further adjusted by Scale X and Scale Y (below). The options are:
 
  - *Best Fit* (default) – the clip is as large as possible without changing the aspect ratio.
  - *Crop* – the aspect ratio of the clip is maintained while the clip is enlarged to fill the entire screen, even if that means some of it will be cropped.
  - *None* – the clip is its original size.
  - *Stretch* – the clip is stretched to fill the entire screen, changing the aspect ratio if necessary.
 
-**Volume Mixing**:
+.. _clip_volume_mixing_ref:
 
- Mixing audio involves adjusting volume levels so that they maintain a good range within each clip, and then adjusting them in proportion to other clips used in the project. The following values are available:
+Volume Mixing
+"""""""""""""
+Mixing audio involves adjusting volume levels so that they maintain a good range within each clip, and then adjusting them in proportion to other clips used in the project. The following values are available:
 
  - **None** - Make no adjustments to volume data before mixing audio
  - **Average** - Automatically divide the volume of each clip based on the # of overlapping clips. For example, 2 overlapping clips would each have 50% volume.
  - **Reduce** - Automatically reduce the clip's volume by 20%, allowing it to mix with other clips, and reducing the likelihood of over-volume loud events.
 
- Consider the following guidelines when adjusting volume levels:
+Consider the following guidelines when adjusting volume levels:
 
  - If you combine particularly loud audio clips on multiple tracks, clipping (a staccato distortion) may occur. To avoid clipping, reduce volume levels.
  - If you need to adjust the volume separately in different parts of a clip (for example, one person’s voice is faint, while later another’s is too loud), you can use keyframes to vary the volume throughout the clip.

doc/playback.rst

@@ -52,6 +52,10 @@ multi-threaded CPU, lots of RAM (memory), and a decent GPU. We have listed many
                        are required. It is recommended to keep reducing the preview window size until you achieve smooth
                        video playback. On a slower computer, the preview window size might need to be very small for
                        real-time previews (i.e. 320 x 240).
+   Profile             Your project profile determines which size (width x height) and frame rate (FPS) are used during both
+                       playback and exporting. For example, if you are using a FHD 1920x1080 sized profile, you can also choose a
+                       smaller profile with the same aspect ratio (``16x9`` in this example), to improve the preview speed
+                       on slower computers. See :ref:`profiles_ref` for more information on available profiles.
    FPS (Frame Rate)    The FPS of your project is also very important, and a large factor for smooth video playback. For
                        example, a 60 FPS video must render twice the number of frames, compared to a 30 FPS video. If
                        you are experiencing slow downs in real-time performance, it can be helpful to reduce your project's
@@ -97,6 +101,8 @@ your issue, here are some additional troubleshooting steps you can take.
    Sample Rate         Ensure that the `Default Audio Sample Rate` and `Default Audio Channels` on the Preview tab of the
                        Preferences window match your hardware. You can also check these settings in the operating system
                        control panel (i.e. Windows Sound Control Panel). See :ref:`preferences_preview_ref`.
+   Volume              Ensure that the volume does not exceed 100% on overlapping clips (such as an audio track combined
+                       with a video track). Lower the volume on individual clips if needed. See :ref:`clip_volume_mixing_ref`.
    Headphones          If you're using headphones, plug them in before starting OpenShot. Launching OpenShot with no
                        speakers, headphones, or valid audio playback device can cause OpenShot to freeze during playback.
    OS Updates          Update your operating system and any pending security updates. Some audio issues, especially