[PATCH v3 4/4] Update documentation regarding 24-bit TTY colors.

["Rami Ylimäki" <rami.ylimaki@vincit.fi>]

From: Rami Ylimäki <rjy@iki.fi>

* doc/misc/efaq.texi: Add instructions on how to enable direct color TTY
  mode.
* etc/NEWS: Mention direct color TTY mode and point to FAQ.
---
 doc/misc/efaq.texi | 33 +++++++++++++++++++++++++++++++++
 etc/NEWS           |  5 +++++
 2 files changed, 38 insertions(+)

diff --git a/doc/misc/efaq.texi b/doc/misc/efaq.texi
index f7a47f8..e9cfe7a 100644
--- a/doc/misc/efaq.texi
+++ b/doc/misc/efaq.texi
@@ -1491,6 +1491,39 @@ exhibits all the colors Emacs knows about on the current display.
 
 Syntax highlighting is on by default since version 22.1.
 
+Emacs 26.1 and later support direct color mode in terminals.  If Emacs
+finds Terminfo capabilities @samp{setb24} and @samp{setf24}, 24-bit
+direct color mode is used.  The capability strings are expected to
+take one 24-bit pixel value as argument and transform the pixel to a
+string that can be used to send 24-bit colors to the terminal.
+
+There aren't yet any standard terminal type definitions that would
+support the capabilities, but Emacs can be invoked with a custom
+definition as shown below.
+
+@example
+$ cat terminfo-24bit.src
+
+# Use colon separators.
+xterm-24bit|xterm with 24-bit direct color mode,
+  use=xterm-256color,
+  setb24=\E[48:2:%p1%@{65536@}%/%d:%p1%@{256@}%/%@{255@}%&%d:%p1%@{255@}%&%dm,
+  setf24=\E[38:2:%p1%@{65536@}%/%d:%p1%@{256@}%/%@{255@}%&%d:%p1%@{255@}%&%dm,
+# Use semicolon separators.
+xterm-24bits|xterm with 24-bit direct color mode,
+  use=xterm-256color,
+  setb24=\E[48;2;%p1%@{65536@}%/%d;%p1%@{256@}%/%@{255@}%&%d;%p1%@{255@}%&%dm,
+  setf24=\E[38;2;%p1%@{65536@}%/%d;%p1%@{256@}%/%@{255@}%&%d;%p1%@{255@}%&%dm,
+
+$ tic -x -o ~/.terminfo terminfo-24bit.src
+
+$ TERM=xterm-24bit emacs -nw
+@end example
+
+Currently there's no standard way to determine whether a terminal
+supports direct color mode.  If such standard arises later on, support
+for @samp{setb24} and @samp{setf24} may be removed.
+
 @node Debugging a customization file
 @section How do I debug a @file{.emacs} file?
 @cindex Debugging @file{.emacs} file
diff --git a/etc/NEWS b/etc/NEWS
index 31b05dd..5cb4d02 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -77,6 +77,11 @@ modern init systems such as systemd, which manage many of the traditional
 aspects of daemon behavior themselves.  '--old-daemon' is now an alias
 for '--daemon'.
 
++++
+** Terminal is initialized to use 24-bit colors if required
+capabilities are found from terminfo.  See the FAQ node 'Colors on a
+TTY' for more information.
+
 \f
 * Changes in Emacs 26.1
 
-- 
2.7.4