Maintainers.txt: clarify Reviewer requirements and responsibilities

Maintainers.txt: clarify Reviewer requirements and responsibilities

The current language for "Package Reviewer" only vaguely hints that
Package Reviewers should be able to provide guidance and directions.
Make this more obvious.

Cc: Andrew Fish <afish@apple.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daude <philmd@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Andrew Fish <afish@apple.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>