Improve before-make-frame-hook and after-make-frame-functions docs.

* lisp/frame.el (before-make-frame-hook)
(after-make-frame-functions):
* doc/lispref/frames.texi (Creating Frames): Mention that these
functions will be run for the initial frame only if added to the
hook in early-init file.  (Bug#38143)

doc/lispref/frames.texi

@@ -171,7 +171,9 @@ usually not run for the initial frame, since Emacs reads the initial
 file only after creating that frame.  However, if the initial frame is
 specified to use a separate minibuffer frame (@pxref{Minibuffers and
 Frames}), the functions will be run for both, the minibuffer-less and
-the minibuffer frame.
+the minibuffer frame.  Alternatively, you can add functions to these
+hooks in your ``early init file'' (@pxref{Init File}), in which case
+they will be in effect for the initial frame as well.
 
 @defvar frame-inherited-parameters
 This variable specifies the list of frame parameters that a newly

lisp/frame.el

@@ -808,12 +808,16 @@ also select the new frame."
     new-frame))
 
 (defvar before-make-frame-hook nil
-  "Functions to run before `make-frame' creates a new frame.")
+  "Functions to run before `make-frame' creates a new frame.
+Note that these functions are usually not run for the initial
+frame, unless you add them to the hook in your early-init file.")
 
 (defvar after-make-frame-functions nil
   "Functions to run after `make-frame' created a new frame.
 The functions are run with one argument, the newly created
-frame.")
+frame.
+Note that these functions are usually not run for the initial
+frame, unless you add them to the hook in your early-init file.")
 
 (defvar after-setting-font-hook nil
   "Functions to run after a frame's font has been changed.")