pru: Document that arguments are not passed to main with -minrt

pru: Document that arguments are not passed to main with -minrt

The minimal runtime has been documented from the beginning to break some
standard features in order to reduce code size, while keeping
the features required by typical firmware programs.  Document one more
imposed restriction - the main() function must take no arguments.

gcc/ChangeLog:

	* doc/invoke.texi (-minrt): Clarify that main
	must take no arguments.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>