Fix help text for --mac-signing-keychain

This change includes help text formatting and minor wording changes as well as described in the comments.

The help text for mac-signing-keychain is:

 --mac-signing-keychain <file path>
          Path of the keychain to search for the signing identity
          (absolute path or relative to the current directory).
          If not specified, the standard keychains are used.

but this is not a path.
It has been fixed in the man pages to say:

 --mac-signing-keychain <keychain name>
          Name of the keychain to search for the signing identity
          If not specified, the standard keychains are used.

this same change should be made to the help text.