docs: locking-related updates

Related to #860

public/static/docs/command-reference/repro.md

@@ -45,6 +45,39 @@ files, intermediate or final results. It saves all the data files, intermediate
 or final results into the <abbr>DVC cache</abbr> (unless `--no-commit` option is
 specified), and updates stage files with the new checksum information.
 
+### Parallel stage execution
+
+Currently, `dvc repro` is not able to parallelize stage execution automatically.
+If you need to do this, you can launch `dvc repro` multiple times manually.
+
+For example, let's say a <abbr>pipeline</abbr> graph looks something like this:
+
+```
+$ dvc pipeline show --ascii result.py
++--------+          +--------+
+| A1.dvc |          | B1.dvc |
++--------+          +--------+
+     *                   *
+     *                   *
+     *                   *
++--------+          +--------+
+| A2.dvc |          | B2.dvc |
++--------+          +--------+
+          *         *
+           **     **
+             *   *
+        +------------+
+        | result.dvc |
+        +------------+
+```
+
+This pipeline consists of two parallel branches (`A` and `B`), and the final
+"result" stage, where the branches merge. To reproduce both branches at the same
+time, you could run `dvc repro A2.dvc` and `dvc repro B2.dvc` at the same time
+(e.g. in separate terminals). After both finish successfully, you can then run
+`dvc repro result.dvc`: DVC will know that both branches are already up-to-date
+and only execute the final stage.
+
 ## Options
 
 - `-f`, `--force` - reproduce a pipeline, regenerating its results, even if no

public/static/docs/user-guide/dvc-files-and-directories.md

@@ -41,6 +41,12 @@ operation:
 
 - `.dvc/lock`: Lock file for the entire DVC project
 
+- `.dvc/tmp`: Directory for miscellaneous temporary files
+
+- `.dvc/tmp/rwlock`: JSON file that contains read and write locks for specific
+  dependencies and outputs, to allow safely running multiple DVC commands in
+  parallel.
+
 ## Structure of cache directory
 
 There are two ways in which the data is stored in <abbr>cache</abbr>: As a