com_ff(): Depreciate "n-" syntax.

com_ff(): Depreciate "n-" syntax.

This syntax was introduced long ago (pre git era) for no good reason.
This patch adjusts the documentation to not mention it any more.

The command handler is modified to use para_atoi32() instead of
sscanf(), with fallback code for the old syntax which also prints a
deprecation warning.

Clarify the documentation by explaining what happens in case the
argument is out of bounds.