master a6f7d0f4e3: ; * src/xterm.c: Improve commentary.

[Po Lu]

branch: master
commit a6f7d0f4e38555080bbdb45dbb414e15d206f548
Author: Po Lu <luangruo@yahoo.com>
Commit: Po Lu <luangruo@yahoo.com>

    ; * src/xterm.c: Improve commentary.
---
 src/xterm.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/xterm.c b/src/xterm.c
index 102447ad7e..7eef2b488b 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -142,14 +142,22 @@ along with GNU Emacs.  If not, see 
<https://www.gnu.org/licenses/>.  */
    draw text in inverse video, and the cursor graphics context is used
    to display the cursor in the most common case.
 
+   N.B. that some of the other window systems supported by use an
+   emulation of graphics contexts to hold the foreground and
+   background colors used in a glyph string, while the some others
+   ports compute those colors directly based on the colors of the
+   string's face and its highlight, but only on X are graphics
+   contexts a data structure inherent to the window system.
+
    COLOR ALLOCATION
 
-   In X, pixel values for colors are not guaranteed to correspond to
-   their individual components.  The rules for converting colors into
-   pixel values are defined by the visual class of each display opened
-   by Emacs.  When a display is opened, a suitable visual is obtained
-   from the X server, and a colormap is created based on that visual,
-   which is then used for each frame created.
+   In (and only in) X, pixel values for colors are not guaranteed to
+   correspond to their individual components.  The rules for
+   converting colors into pixel values are defined by the visual class
+   of each display opened by Emacs.  When a display is opened, a
+   suitable visual is obtained from the X server, and a colormap is
+   created based on that visual, which is then used for each frame
+   created.
 
    The colormap is then used by the X server to convert pixel values
    from a frame created by Emacs into actual colors which are output
@@ -202,6 +210,10 @@ along with GNU Emacs.  If not, see 
<https://www.gnu.org/licenses/>.  */
    which is used to determine the color values for given pixel
    values.
 
+   In other window systems supported by Emacs, color allocation is
+   handled by the window system itself, to whom Emacs simply passes 24
+   (or 32-bit) RGB values.
+
    OPTIONAL FEATURES
 
    While X servers and client libraries tend to come with many