pgsql: Fix actively-misleading comments about the contents of struct pg

Fix actively-misleading comments about the contents of struct pg_tm.

pgtime.h documented the PG interpretation of tm_mon right alongside
the POSIX interpretation of tm_year, with no hint that neither
comment was correct throughout our code.

Perhaps someday we ought to switch to using two separate struct
definitions to provide a clearer indication of which semantics are
in use where.  But I fear the tedium-versus-safety-gain tradeoff
would not be very good.

Discussion: https://postgr.es/m/CAJ7c6TOMG8zSNEZtCn5SPe+cCk3Lfxb71ZaQwT2F4T7PJ_t=KA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/89dba59590fdd03799a47daf8019890d4324fbcf

Modified Files
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src/include/pgtime.h | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)