main/pbx - documentation - enhance 'core show hints' and 'core show hint' help text

Adds descriptive help text to 'core show hints' and 'core show hint'. The text describes the various columns for the sake of clarity.

ASTERISK-23764
Review: https://reviewboard.asterisk.org/r/3610/
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Merged revisions 415998 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@415999 65c4cc65-6c06-0410-ace0-fbb531ad65f3