Is there a bpo page or some sort that discusses why the imperative mood is used over the indicative mood other than convention of PEP 257? I found this (https://mail.python.org/pipermail/tutor/2012-May/089584.html) question and answer that seems to make sense, but it's not quite an official source.
If the reason for using imperative mood in docstrings doesn't apply to documentation, then I think there wouldn't be any problems in changing it. One thing raised in the mail thread was that 'Returns ...' isn't strictly grammatical, so perhaps that's something to consider.