Docs: update default branch based on VCS providers
Mention Git and Mercurial only becase Bazaar and SVN only add noise at this point.
@@ -4,7 +4,8 @@ Versioned Documentation
Read the Docs supports multiple versions of your repository.
On initial import,
we will create a ``latest`` version.
-This will point at the default branch for your VCS control: ``master``, ``default``, or ``trunk``.
+This will point at the default branch defined in your VCS control
+(by default, ``main`` on Git and ``default`` in Mercurial).
If your project has any tags or branches with a name following `semantic versioning <https://semver.org/>`_,
we also create a ``stable`` version, tracking your most recent release.
