Add c-flag to tail(1) and refactor manpage

and mark it as finished in README.

tail.1

@@ -7,7 +7,7 @@
 .Sh SYNOPSIS
 .Nm
 .Op Fl f
-.Op Fl n Ar num
+.Op Fl c Ar num | Fl n Ar num
 .Op Ar file ...
 .Sh DESCRIPTION
 .Nm
@@ -20,19 +20,28 @@ is given,
 reads from stdin.
 .Sh OPTIONS
 .Bl -tag -width Ds
+.It Fl c Ar num | Fl n Ar num
+Display
+.Ar num
+last characters | lines. If
+.Ar num
+begins with '+' it is used as an offset from the beginning of each
+.Ar file .
+If
+.Ar num
+begins with '-' it is handled the same as if no sign was given.
 .It Fl f
 If one
 .Ar file
 is specified, append lines to output as
 .Ar file
 grows.
-.It Fl n Ar lines
-Display
-.Ar num
-lines. If
-.Ar num
-begins with '+' it is used as an offset from the beginning of each
-.Ar file .
 .El
 .Sh SEE ALSO
 .Xr head 1
+.Sh STANDARDS
+The
+.Nm
+utility is compliant with the
+.St -p1003.1-2008
+specification.