Overall more reliable docstring format detection

Overall more reliable docstring format detection

I added several convenient methods for detection docstring format
in DocStringUtil. One of them - parse(String, PsiElement?) fallbacks to
docstring format specified in settings if it was unable to detect
docstring format solely from text. I replaced existing usages of
#parse(String) with this method. For PLAIN format it also returns
special PlainDocString type that can only extract summary and
description, e.g. for documentation popups.