Improve documentation of 'file-local-name' and related APIs

Improve documentation of 'file-local-name' and related APIs

* doc/lispref/files.texi (Unique File Names)
(Magic File Names, File Name Expansion): Improve documentation
of the "local part" of a remote file name.
* doc/lispref/processes.texi (Synchronous Processes)
(Asynchronous Processes): State explicitly that program and
file names passed to functions that start remote processes
need to be relative or obtained by 'file-local-name'.

* lisp/files.el (file-local-name):
* lisp/simple.el (start-file-process, process-file): Improve
the documentation of the "local part" of a remote file name,
and its use in APIs that start remote processes.