Minor improvements for 'pgtk' documentation

* src/dispnew.c (syms_of_display) <initial-window-system>
<window-system>:
* src/frame.c (Fwindow_system):
* doc/lispref/display.texi (Defining Faces, Window Systems):
* doc/lispref/frames.texi (Frames): Mention/explain 'pgtk'/'haiku'.
* doc/lispref/commands.texi (Misc Events): Fix @example and
markup.

doc/lispref/commands.texi

@@ -2139,19 +2139,29 @@ On X, @var{arg} is a string describing some text to place behind the
 cursor.  It can be @code{nil}, which means to remove any text
 previously displayed.
 
-On PGTK, @var{arg} is a list of strings with color information.  Its
-structure is as follows:
+On PGTK frames (@pxref{Frames}), @var{arg} is a list of strings with
+information about their color and underline attributes.  It has the
+following form:
 
 @example
-   (
-     (TEXT (ul . COLOR) (bg . COLOR) (fg . COLOR))
-     ; ...
+@group
+   ((@var{string1}
+     (ul . @var{underline-color})
+     (bg . @var{background-color})
+     (fg . @var{foreground-color}))
+    (@var{string2}
+     (ul . @var{underline-color})
+     (bg . @var{background-color})
+     (fg . @var{foreground-color}))
+    @dots{}
    )
+@end group
 @end example
 
-Color information can be omitted.  @code{COLOR} of @code{ul} can be
-@code{t} or a string.  @var{arg} can be @code{nil}, which means to
-remove any text previously displayed.
+Color information can be omitted, leaving just the text of the
+strings.  @var{underline-color} can be @code{t}, meaning underlined
+text with default underline color, or it can be a string, the name of
+the color to draw the underline.
 
 This is a special event (@pxref{Special Events}), which normally
 should not be bound by the user to any command.  Emacs will typically

doc/lispref/display.texi

@@ -2802,7 +2802,7 @@ apply to.  Here are the possible values of @var{characteristic}:
 The kind of window system the terminal uses---either @code{graphic}
 (any graphics-capable display), @code{x}, @code{pc} (for the MS-DOS
 console), @code{w32} (for MS Windows 9X/NT/2K/XP), @code{haiku} (for
-Haiku), @code{pgtk} (for GTK), or @code{tty} (a non-graphics-capable
+Haiku), @code{pgtk} (for pure GTK), or @code{tty} (a non-graphics-capable
 display).  @xref{Window Systems, window-system}.
 
 @item class
@@ -8386,6 +8386,8 @@ GNUstep and macOS).
 Emacs is displaying the frame using MS-DOS direct screen writes.
 @item haiku
 Emacs is displaying the frame using the Application Kit on Haiku.
+@item pgtk
+Emacs is displaying the frame using pure GTK facilities.
 @item nil
 Emacs is displaying the frame on a character-based terminal.
 @end table

doc/lispref/frames.texi

@@ -60,6 +60,8 @@ The frame is displayed on a GNUstep or Macintosh Cocoa graphical
 terminal.
 @item pc
 The frame is displayed on an MS-DOS terminal.
+@item pgtk
+The frame is displayed using pure GTK facilities.
 @end table
 @end defun
 

src/dispnew.c

@@ -6662,6 +6662,8 @@ The value is a symbol:
  `w32' for an Emacs frame that is a window on MS-Windows display,
  `ns' for an Emacs frame on a GNUstep or Macintosh Cocoa display,
  `pc' for a direct-write MS-DOS frame.
+ `pgtk' for an Emacs frame using pure GTK facilities.
+ `haiku' for an Emacs frame running in Haiku.
 
 Use of this variable as a boolean is deprecated.  Instead,
 use `display-graphic-p' or any of the other `display-*-p'
@@ -6675,6 +6677,8 @@ The value is a symbol:
  `w32' for an Emacs frame that is a window on MS-Windows display,
  `ns' for an Emacs frame on a GNUstep or Macintosh Cocoa display,
  `pc' for a direct-write MS-DOS frame.
+ `pgtk' for an Emacs frame using pure GTK facilities.
+ `haiku' for an Emacs frame running in Haiku.
 
 Use of this variable as a boolean is deprecated.  Instead,
 use `display-graphic-p' or any of the other `display-*-p'

src/frame.c

@@ -277,6 +277,8 @@ The value is a symbol:
  `w32' for an Emacs frame that is a window on MS-Windows display,
  `ns' for an Emacs frame on a GNUstep or Macintosh Cocoa display,
  `pc' for a direct-write MS-DOS frame.
+ `pgtk' for an Emacs frame using pure GTK facilities.
+ `haiku' for an Emacs frame running in Haiku.
 
 FRAME defaults to the currently selected frame.