help: Make terminology around tagging more consistent

The code and various things like the README refer to this as ‘tagging’,
but the documentation referred to it as ‘highlighting’. The latter is
quite a general word, so use the former more consistently.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #15
2 jobs for 15-docs in 3 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #626713
build-and-test

00:03:49

passed #626714
flatpak
flatpak

00:00:42