Update forc plugin docs with a note on multi-command plugins

[mitchmindtree]

Previously, a user could be mistaken for thinking a single plugin could only ever provide a single command. This note should help clarify that a plugin can provide more than one command by specifying multiple [[bin]] targets and uses the forc-client plugin as an example.

We should land #2440 first so that the forc-client plugin reference makes sense.