Extract --flag_alias from bazelrc and use docstrings of target to provide better bazel --help

[aiuto]

Currently, Bazel flags show up in bazel --help, but user defined flags do not.
As more code moves from native Bazel to Starlark, important flags for are likely to acquire --flag_aliases in bazelrc to make them easier to use. This is especially true for flags to core language rules like --copt Having the documentation for those as part of --help would prevent loss of documentation as they migrate.

This would build on the basic ability to document targets from #108