Adjust output of --help text to mirror command line changes (#5334)

This commit adjusts the output of running `mypy --help` to mirror the
changes made in #5333.

Specifically, it rearranges the order of the flags slightly and replaces
the summary of the flags at the top with a short description of how to
use mypy along with links for learning more.

It also makes every group title and flag help text start with a capital.

Author: Michael0x2a