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.
1 job for emacs-26 in 6 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #404
test

00:00:06

 
Name Stage Failure
failed
test Test There has been a runner system failure, please try again
Running with gitlab-runner 11.4.2 (cf91d5e1)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:de8b49d4b0b316649307dcdda2ca0c6127f8ccdaa9c3fda6fdb21cb237cfafd8 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: open /var/run/docker/libcontainerd/containerd/b0e86d9969fdfa013f0076ea6feb7974fbd0ad1a7991507f45e3ee9d69fd1610/init/shim-log.json: no such file or directory (executor_docker.go:1015:1s)