Improve documentation.
Use "P4" for this program, "p4" for the Perforce command-line program, and "Perforce" for the revision control system.
Use the one-line descriptions from "p4 help" in the docstrings.
Fix bugs in p4-set-client-name and p4-arg-string-completion.
No need for p4-set-my-clients: use customize instead.