Documentation/process: tweak pgp maintainer guide

Based on the feedback provided:

- Uniformly use lowercase k in "Linux kernel"
- Give a one-sentence explanation of what subkeys are
- Explain what signed commits might be useful for even if upstream
  developers do not use them for much of anything
- Admonish to set up gpg-agent if signed commits are turned on in
  git config
- Fix a typo reported by Luc Van Oostenryck

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>