Clarify manual and `add-hook' doc string about buffer-local hooks

doc/emacs/ChangeLog

+2011-07-13  Lars Magne Ingebrigtsen  <larsi@gnus.org>
+
+	* custom.texi (Hooks): Mention buffer-local hooks (bug#6218).
+
 2011-07-13  Glenn Morris  <rgm@gnu.org>
 
 	* dired.texi (Dired Enter): Mention --dired.  (Bug#9039)

doc/emacs/custom.texi

@@ -820,6 +820,12 @@ the versions you added will remain in the hook variable together.  You
 can clear out individual functions by calling @code{remove-hook}, or
 do @code{(setq @var{hook-variable} nil)} to remove everything.
 
+@cindex buffer-local hooks
+  If the hook variable is buffer-local, the buffer-local variable will
+be used instead of the global variable.  However, if the buffer-local
+variable contains the element @code{t}, the global hook variable will
+be run as well.
+
 @node Locals
 @subsection Local Variables
 

lisp/ChangeLog

 2011-07-13  Lars Magne Ingebrigtsen  <larsi@gnus.org>
 
+	* subr.el (add-hook): Clarify section about buffer-local hooks
+	(bug#6218).
+
 	* dired.el (dired-flagged): Clarify doc string (bug#6117).
 
 2011-07-13  Juanma Barranquero  <lekktu@gmail.com>

lisp/subr.el

@@ -1262,10 +1262,10 @@ unless the optional argument APPEND is non-nil, in which case
 FUNCTION is added at the end.
 
 The optional fourth argument, LOCAL, if non-nil, says to modify
-the hook's buffer-local value rather than its default value.
-This makes the hook buffer-local if needed, and it makes t a member
-of the buffer-local value.  That acts as a flag to run the hook
-functions in the default value as well as in the local value.
+the hook's buffer-local value rather than its global value.
+This makes the hook buffer-local, and it makes t a member of the
+buffer-local value.  That acts as a flag to run the hook
+functions of the global value as well as in the local value.
 
 HOOK should be a symbol, and FUNCTION may be any valid function.  If
 HOOK is void, it is first set to nil.  If HOOK's value is a single