Doc: fix obsolete info about allowed range of TZ offsets in timetz.
We've allowed UTC offsets up to +/- 15:59 since commit
cd0ff9c0f, but
that commit forgot to fix the documentation about timetz.
Per bug #16571 from osdba.
Discussion: https://postgr.es/m/16571-
eb7501598de78c8a@postgresql.org