GRegex: update documentation for partial matching

GRegex: update documentation for partial matching

The restrictions on partial matching no longer apply with PCRE >= 8.00.

The pcrepartial manpage contains the "FORMERLY RESTRICTED PATTERNS"
section:

"For  releases  of PCRE prior to 8.00, because of the way certain
internal optimizations were implemented in the pcre_exec() function, the
PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used
with all patterns. From release 8.00 onwards, the restrictions no
longer  apply,  and  partial  matching  with  can  be requested for any
pattern."

https://bugzilla.gnome.org/show_bug.cgi?id=704250