Add additional notes regarding srand and forking

Add additional notes regarding srand and forking

perldoc -f srand states that typical use requires no srand() to be
called.  This is true with the exception of forking where you may not
want the same seed across various child processes (i.e. mod_perl).
This patch simply adds a note reminding the reader of this fact and
more specifically states that srand should only be called once per
*process* (instead of the previous language of once per *script*).

Signed-off-by: Brian Phillips <bphillips@cpan.org>