Document default process sentinel more prominently

Document default process sentinel more prominently

* doc/lispref/processes.texi (Asynchronous Processes): Mention the
defaults for process filter and sentinel.  Provide cross-references.
(Process Information): Provide cross-references to where filters
and sentinels are described.
(Filter Functions): Add an index entry for "default filter".
(Sentinels): Add a few status messages not documented previously.
Resolve the "killed" confusion.  Document and describe the default
sentinel.  (Bug#22220)