Code and docs review for cube kNN support.

Code and docs review for cube kNN support.

Commit 33bd250f6c4cc309f4eeb657da80f1e7743b3e5c could have done with
some more review:

Adjust coding so that compilers unfamiliar with elog/ereport don't complain
about uninitialized values.

Fix misuse of PG_GETARG_INT16 to retrieve arguments declared as "integer"
at the SQL level.  (This was evidently copied from cube_ll_coord and
cube_ur_coord, but those were wrong too.)

Fix non-style-guide-conforming error messages.

Fix underparenthesized if statements, which pgindent would have made a
hash of, and remove some unnecessary parens elsewhere.

Run pgindent over new code.

Revise documentation: repeated accretion of more operators without any
rethinking of the text already there had left things in a bit of a mess.
Merge all the cube operators into one table and adjust surrounding text
appropriately.

David Rowley and Tom Lane