[llvm-extract] Update the help message for group extraction feature

Summary:
https://reviews.llvm.org/D60973 exposed the group extraction feature of
the BlockExtractor to llvm-extract.
However, the help message was not updated, so users might not be able to
know how to use this feature without looking into history/commits.

This patch just update the help message to show how to use this group
extraction feature.

Reviewers: qcolombet, volkan

Reviewed By: qcolombet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66438

llvm-svn: 369438