Tidy up a few cosmetic issues related to pg_basebackup.

Tidy up a few cosmetic issues related to pg_basebackup.

Commit 0ad8032910d5eb8efd32867c45b6a25c85e60f50 failed to update
the pg_basebackup documentation to mention that "client-" or
"server-" can now be prepended to the compression method name. Fix
it there, and also in the --help output that you get from running
the binary.

Also in the documentation, there's an old issue that the arguments to
--checkpoint shouldn't be marked as parameters, because "fast" and
"spread" are literal strings. Fix that too.

Dagfinn Ilmari Mannsåker, partly as per a report from
Shinoda Noriyoshi.

Discussion: http://postgr.es/m/PH7PR84MB1885C1CF433057807551172BEE5F9@PH7PR84MB1885.NAMPRD84.PROD.OUTLOOK.COM