pgsql: Doc: explicitly point out that enum values can't be dropped.

Doc: explicitly point out that enum values can't be dropped.

This was not stated in so many words anywhere.  Document it to make
clear that it's a design limitation and not just an oversight or
documentation omission.

Discussion: https://postgr.es/m/152089733343.1222.6927268289645380498@wrigleys.postgresql.org

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b7fbd3f4838631f7279704638caa74b974f22371

Modified Files
--------------
doc/src/sgml/datatype.sgml | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)