Add some notes about why "ALTER TYPE enum DROP VALUE" is hard.

Add some notes about why "ALTER TYPE enum DROP VALUE" is hard.

In hopes of putting these where any would-be implementer is sure to
find them, make a placeholder grammar production for ALTER DROP VALUE
and put them there.  This is really just a docs patch, though.

Vik Fearing, with a bit more wordsmithing by me

Discussion: https://postgr.es/m/9fffd149-da0f-0c9c-6745-731fb688642a@postgresfriends.org