Add detailed documentation about lock files

Add detailed documentation about lock files

* doc/emacs/files.texi (Interlocking): Point user at detailed
file locking description in lisp reference manual.  Add index
entry for '.#' to improve disoverability of information about locking.

* doc/lispref/files.texi (File Locks): Describe in detail what
the form of the lock file is.  Add index entry for '.#' to
improve disoverability of information about locking.

* src/filelock.c (create-lockfiles): Add cross reference to
file locking in user manual and to 'lock-buffer'.  Add string
'.#' to help users find the doc string.