docs/faq: Improve answer regarding old branches/tags.

docs/faq.rst

@@ -24,10 +24,16 @@ Then it copies the data for each version into separate temporary directories, bu
 The :file:`conf.py` file from the currently checked out branch will be used to build old versions, so it's not necessary to make changes old branches or tags to add support for ``sphinx-multiversion``.
 This also means that theme improvements, template changes, etc. will automatically be applied to old versions without needing to add commits.
 
+
 Do I need to make changes to old branches or tags?
 ==================================================
 
-No, you don't. ``sphinx-multiversion`` will always use the :file:`conf.py` file from you currently checked out branch.
+No, you don't. ``sphinx-multiversion`` will always use the :file:`conf.py` file from your currently checked out branch.
+
+The downside is that this behaviour restricts the kinds of changes you may do to your configuration, because it needs to retain compatibility with old branches.
+For example, if your :file:`conf.py` file hardcodes a path (e.g. for opening a file), but that file does not exist in some older branches that you want to build documentation for, this will cause issues.
+In these cases you will need to add a check if a file actually exists and adapt the path accordingly.
+
 
 What are the license terms of ``sphinx-multiversion``?
 ======================================================