Document RelationGetIndexAttrBitmap better

Document RelationGetIndexAttrBitmap better

Commit 19d8e2308bc5 changed the list of set-of-columns that can be
returned by RelationGetIndexAttrBitmap, but didn't update its
"documentation".  That was pretty hard to read already, so rewrite to
make it more comprehensible, adding the missing values while at it.

Backpatch to 16, like that commit.

Discussion: http://postgr.es/m/20230809091155.7c7f3gttjk3dj4ze@alvherre.pgsql
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>