This change is three fold but I'm willing to split this in parts if necessary.
* Add a small description before the EXAMPLES that are already in the man page.
It is always better to explicitely state what we are trying to show instead
of having the user guess what the example is doing. Even for simple cases.
* Add two more examples to show usage of `-s`, `-t` and `-f`
*`mandoc -Tlint` reports irrelevant use of `.Tn` so remove them since
according to `mdoc(7)` it is there only for compatibility and should not be
used in new manuals.
I did not run any `atf(7)` tests but having a look at the script in `test/` I
can't see anything related to the manual page.