docs: Emphasize that pip may break existing packages

Related to #7744 .

docs/html/user_guide.rst

@@ -198,7 +198,7 @@ In practice, there are 4 common uses of Requirements files:
          py -m pip install -r requirements.txt
 
 2. Requirements files are used to force pip to properly resolve dependencies.
-   As it is now, pip `doesn't have true dependency resolution
+   pip 20.2 and earlier `doesn't have true dependency resolution
    <https://github.com/pypa/pip/issues/988>`_, but instead simply uses the first
    specification it finds for a project. E.g. if ``pkg1`` requires
    ``pkg3>=1.0`` and ``pkg2`` requires ``pkg3>=1.0,<=2.0``, and if ``pkg1`` is
@@ -1430,12 +1430,13 @@ time to fix the underlying problem in the packages, because pip will
 be stricter from here on out.
 
 This also means that, when you run a ``pip install`` command, pip only
-considers the packages you are installing in that command, and may
-break already-installed packages. It will not guarantee that your
+considers the packages you are installing in that command, and **may
+break already-installed packages**. It will not guarantee that your
 environment will be consistent all the time. If you ``pip install x``
 and then ``pip install y``, it's possible that the version of ``y``
 you get will be different than it would be if you had run ``pip
-install x y`` in a single command. We would like your thoughts on what
+install x y`` in a single command. We are considering changing this
+behavior (per :issue:`7744`) and would like your thoughts on what
 pip's behavior should be; please answer `our survey on upgrades that
 create conflicts`_.