docs: quickly-build-trimmed-linux: various small fixes and improvements

* improve the short description of localmodconfig in the step-by-step
  guide while fixing its broken first sentence

* briefly mention immutable Linux distributions

* use '--shallow-exclude=v6.0' throughout the document

* instead of "git reset --hard; git checkout ..." use "git checkout
  --force ..." in the step-by-step guide: this matches the TLDR and is
  one command less to execute. This led to a few small adjustments to
  the text and the flow in the surrounding area.

* fix two thinkos in the section explaining full git clones

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/6f4684b9a5d11d3adb04e0af3cfc60db8b28eeb2.1684140700.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <corbet@lwn.net>