pgsql: Clean up code comments for pg_wchar_tbl

Clean up code comments for pg_wchar_tbl

The documentation for the functions in the fields of pg_wchar_tbl was
spread out all over the place and was missing a lot of detail. Move
and expand this at the definition of the struct. (See also struct
TableAmRoutine in access/tableam.h for inspiration.)

Also remove unnecessary typedefs, except where they were used
elsewhere as well. This indirection makes the code documentation
harder to follow. And some of those typedefs had nonsensical names,
like *_converter for functions that didn't do any converting.

Reviewed-by: Greg Burd <greg(at)burd(dot)me>
Discussion: https://www.postgresql.org/message-id/flat/637d73f4-e088-4ead-b9e6-1ddb4cd4693f%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3ba886b35a2804dcd275868a19135a31ceb81386

Modified Files
--------------
src/backend/utils/mb/mbutils.c | 10 ++--
src/common/wchar.c | 66 +++------------------
src/include/mb/pg_wchar.h | 125 ++++++++++++++++++++++++++++++++-------
src/tools/pgindent/typedefs.list | 8 +--
4 files changed, 117 insertions(+), 92 deletions(-)