pgsql: Create the beginnings of internals documentation for the regex c

Create the beginnings of internals documentation for the regex code.

Create src/backend/regex/README to hold an implementation overview of
the regex package, and fill it in with some preliminary notes about
the code's DFA/NFA processing and colormap management. Much more to
do there of course.

Also, improve some code comments around the colormap and cvec code.
No functional changes except to add one missing assert.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/27af91438b68f46f4015853b6f75c6f5c3a8650c

Modified Files
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src/backend/regex/README | 291 +++++++++++++++++++++++++++++++++++++++++
src/backend/regex/regc_cvec.c | 13 ++-
src/backend/regex/regcomp.c | 1 +
src/include/regex/regguts.h | 54 ++++++--
4 files changed, 343 insertions(+), 16 deletions(-)