Add documentation about unsupported syntax without --fast-parser

[JukkaL]

The default parser doesn't support some Python 3 syntax, and this probably affects many users and it's unclear what to do about it. It would be good to at least mention this in the section "Dealing with common issues".

[JukkaL]

If we can make the fast parser the default for the next release, this won't be important. However, tagging this a high priority so that we won't lose track of this in case that gets postponed.

[JukkaL]

The fast parser is now on by default and the old parser will be dropped in the next release.