logcat: Note in help that regexps are Perl-compatible

The expressions accepted by the '-e'/'--grep' command line option are
PCRE:

https://android.googlesource.com/platform/system/core/+/master/logcat/logcat.cpp#1018

Say so in the help string.

Change-Id: I7f6af130d5e39570bb49310a928f5d495e5b269b