add hint about `starts_for_each_pattern` to `regex_automata::MatchErrorKind::UnsupportedAnchored` documentation

[BurntSushi]

I’ve been there, but the comment is a bit sparse:

An error indicating that a particular type of anchored search was requested, but that the regex engine does not support it.

Just saying that the most common cases/mandatory conditions for DFA/NFA are to have the starts_for_each_pattern might help (I assume that starts_for_each_pattern is mandatory for Anchored(PatternId(_)) variant; I don’t know what’s necessary for the Yes one)

Originally posted by @gagbo in #1261 (reply in thread)