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

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_alias`es 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

Start by reading the existing target documentation work from #108 and the behavior of `bazel --help`. Trace how `--flag_alias` entries in `bazelrc` and target docstrings could contribute to help output; done means user-defined aliased flags are documented there without losing the existing native flag help.

Written by the indexing model from the issue text.