pg_verify_checksums review

In looking over pg_verify_checksums I found a few small things that I think
would improve on it:

* pg_verify_checksums was placed in the Client Utils section in the docs.
Since it requries physical access to the cluster datafiles it seems to belong
in the Server Utils section.

* The -D option and supported environment variable wasn’t documented.

* Only -D is supported for specifying the data directory, but most all other
utilities also support --pgdata on top of -D. To present a consistent user
interface we should probably support --pgdata in pg_verify_checksums as well.

The latter is I assume too invasive as we are past the freeze, but the first
two docs patches would make sense in 11 IMO as they document whats in the tree.

The attached patches fixes the above mentioned things (I don’t have a docs
toolchain working right now so the docs patches are best effort).

cheers ./daniel