Minor code review for parse_phrase_operator().

Fix its header comment, which described the old behavior of the <N>
phrase distance operator; we missed updating that in commit 028350f6.
Also, reset errno before strtol() call, to defend against the possibility
that it was already ERANGE at entry.  (The lack of complaints says that
it generally isn't, but this is at least a latent bug.)  Very minor
stylistic improvements as well.

Victor Drobny noted the obsolete comment, I noted the errno issue.
Back-patch to 9.6 where this code was added, just in case the errno
issue is a live bug in some cases.

Discussion: https://postgr.es/m/2b5382fdff9b1f79d5eb2c99c4d2cbe2@postgrespro.ru