Flymake backends must check proc obsoleteness in source buffer

Flymake backends must check proc obsoleteness in source buffer

Flymake backends that rely on sentinels running asynchronously and
checking buffer-local variables that hold running processes should
check for obsoleteness in the correct buffers, otherwise a (harmless)
warning might be triggered.

* doc/misc/flymake.texi (An annotated example backend):
Check buffer-local proc in the current buffer.

* lisp/progmodes/elisp-mode.el (elisp-flymake-byte-compile):
Check buffer-local proc in the current buffer.