docs: document meaning of [U.], [U*], [A.], [A*], and [R.]

[heitorPB]

What is the meaning of . and * in the CHANGED, ADDED, and REMOVED sections of nh os switch .?

[faukah]

TL;DR:

# Package install state (by pname):
# I: Installed (unchanged)
# A: Added to closure
# R: Removed from closure
# U: Upgrade
# D: Downgrade
# C: Changed versions or selection state

# Package selection state (by pname):
# +: Package is newly selected in environment.systemPackages.
# -: Package is newly unselected.
# *: Package is selected; state unchanged.
# .: Not in environment.systemPackages; dependency.
# l: Selected package set only available on the left; not selected there.
# L: Selected package set only available on the left; selected there.
# r: Selected package set only available on the right; not selected there.
# R: Selected package set only available on the right; selected there.

The output is dix, which uses nvd's format.
It is not documented in the docs in either project but you can see the meaning here.
I'll document it properly in both nh and dix.