Move ATExecAlterColumnGenericOptions away from where it was unthinkingly
dropped, in the middle of a lot of ALTER COLUMN TYPE code.  I don't have
any high principles about where to put it instead, so let's just put it
after ALTER COLUMN TYPE and before ALTER OWNER, matching existing
decisions about how to order related code stanzas.

Also add the minimal function header comment that the original author
was too cool to bother with.

Along the way, upgrade header comments for nearby ALTER COLUMN TYPE
functions.

Discussion: https://postgr.es/m/14787.1561403130@sss.pgh.pa.us