pgsql: Add comments re text <-> bytea internal equivalence in convert

Log Message:
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Add comments re text <-> bytea internal equivalence in convert routines.

Modified Files:
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pgsql/src/backend/utils/mb:
mbutils.c (r1.65 -> r1.66)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c?r1=1.65&amp;r2=1.66)