Doc: rm `-none` from constraint section, make `installed` prominent
- `PACKAGE -none` as already been removed in 3.4 as valid constraint.
(I guess this is now flag syntax.)
- Important constraint `installed` was just buried in an example, I made
it stand out more in a bullet point.
Same for `source`, even though this is an exotic constraint.
- Highlight constraint `installed` in connection with `allow-newer`.
- Group `test` and `bench` constraint with the `+/-FLAG` constraints.
Fixup: Apply concrete suggestions from code review
Fixes suggested by @ulysses4ever, thanks@
Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>
(cherry picked from commit
d9452a455b61d7375badd330b43079d87f8573be)