seq(1): complete EXAMPLES section

• Add a small description before the EXAMPLES that are already in the man page to explicitely state what we are trying to show instead of having the user guess what the example is doing.

• 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.

Approved by: 0mp@