bug#54338: enhancement (documentation): explain permission tests in "test"

[Ulrich Windl]

I noticed that "test -r file" returns success when called as root for a file
with these permissions:
"--w-------" (see also https://stackoverflow.com/q/71435657/6607497)

The documentation simply states:
‘-r FILE’
     True if FILE exists and read permission is granted.

Doing an strace it seems stat() is used to check the permissions (well, what
about ACLs, just in case?)

I think there should be a better explanatiomn how the permission tests work,
especially when called as "root".

(Report based on coreutils-8.32-150300.3.5.1 from openSUSE Leap 15.3)