Fix process filter documentation (Bug#13400)

* doc/lispref/processes.texi (Asynchronous Processes): Note that input
may read when sending data as well.
(Output from Processes): Note that functions which send data may also
trigger reading from processes.
(Input to Processes, Filter Functions): Note that filter functions may
be called recursively.
1 job for emacs-26 in 26 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #3040


Name Stage Failure
test Test
Makefile:285: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:255: recipe for target 'check' failed
make[1]: *** [check] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:951: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1