Improve process doc. with respect to handling of large input (Bug#33191)

Improve process doc. with respect to handling of large input (Bug#33191)

* src/process.c (Fprocess_send_region, Fprocess_send_string):
Document that process input longer than the process input
buffer may be split into bunches.  Remove an outdated
reference to a 500 character split boundary.
* doc/lispref/processes.texi (Asynchronous Processes): Remove
mention of "stray character injections" in PTY processes.  See
also the comment about ICANON in src/sysdep.c, function
child_setup_tty.