Deprecate the pants() wrapper from around pants() targets. Update docs, and add a deprecation message:

  *** pants() wrapper is obsolete and will be removed in a future release. See http://pantsbuild.github.io/build_files.html ***

Testing done:

Compile pants, saw new warning.  Scanned the docs locally running build-support/bin/publish_docs.sh and made sure url in warning worked.

Bugs closed: 486

Reviewed at: https://rbcommons.com/s/twitter/r/903/