Clarify documentation of face attribute functions

* doc/lispref/display.texi (Attribute Functions): Make sure to
document that an omitted FRAME argument means the same as nil.
(Bug#24879)

doc/lispref/display.texi

@@ -2548,7 +2548,7 @@ modifying the attributes of a named face.
 This function returns the value of the @var{attribute} attribute for
 @var{face} on @var{frame}.
 
-If @var{frame} is @code{nil}, that means the selected frame
+If @var{frame} is omitted or @code{nil}, that means the selected frame
 (@pxref{Input Focus}).  If @var{frame} is @code{t}, this function
 returns the value of the specified attribute for newly-created frames
 (this is normally @code{unspecified}, unless you have specified some
@@ -2644,8 +2644,8 @@ created frames.
 
   The following commands and functions mostly provide compatibility
 with old versions of Emacs.  They work by calling
-@code{set-face-attribute}.  Values of @code{t} and @code{nil} for
-their @var{frame} argument are handled just like
+@code{set-face-attribute}.  Values of @code{t} and @code{nil} (or
+omitted) for their @var{frame} argument are handled just like
 @code{set-face-attribute} and @code{face-attribute}.  The commands
 read their arguments using the minibuffer, if called interactively.
 
@@ -2714,7 +2714,8 @@ name used for @var{character}.
 @defun face-foreground face &optional frame inherit
 @defunx face-background face &optional frame inherit
 These functions return the foreground color (or background color,
-respectively) of face @var{face}, as a string.
+respectively) of face @var{face}, as a string.  If the color is
+unspecified, they return @code{nil}.
 @end defun
 
 @defun face-stipple face &optional frame inherit