document ~| format modifier to place a tab stop at this position

This works best when used in tandem with ~N+, since it is currently
equivalent to ~0| and does not take the actual text position into account.
However, when using relative positions, this works as intended.

src/prolog/lib/format.pl

@@ -28,6 +28,7 @@
            if N is 0 or omitted, no decimal point is used.
      ~ND   like ~Nd, separating digits to the left of the decimal point
            in groups of three, using the character "," (comma)
+     ~|    place a tab stop at this position
      ~N|   where N is an integer: place a tab stop at text column N
      ~N+   where N is an integer: place a tab stop N characters
            after the previous tab stop (or start of line)