Some doc updates for default process sentinels and filters not being nil

Some doc updates for default process sentinels and filters not being nil

* doc/lispref/internals.texi (Process Internals):
* doc/lispref/processes.texi (Deleting Processes, Output from Processes)
(Process Buffers, Filter Functions, Accepting Output, Sentinels)
(Network, Network Servers, Network Processes, Serial Ports):
Filters and sentinels can no longer be nil.

* doc/lispref/elisp.texi (Top): Menu update.

* doc/misc/flymake.texi (Starting the syntax check process): Grammar fix.

* doc/misc/tramp.texi (External packages): Grammar fix.
Reword for default sentinel not being nil any more.

* src/process.c (Fprocess_buffer, Faccept_process_output)
(Finternal_default_process_filter, Finternal_default_process_sentinel):
Doc fixes.