runtime(doc): use standard SGR format at :h xterm-true-color

By default, Vim uses the non-standard, but widely supported, legacy
xterm/Konsole format for setting "direct colors" with set setaf and
setbf escape codes, which use semicolons as separators.

The documentation for xterm-true-color mentions that, as an alternative,
users can set alternative sequences that use colons instead of
semicolons. This format, though, isn't standard and it is unclear how
widely supported it is; it was added by xterm patch 282 due to a
misinterpretation of the ISO 8613-6 (ITU T.416) standard, and was later
changed to the format suggested by this patch, which is the one
specified in the standard.

Today, looking at ncurses' [terminfo], it seems that all terminal
emulators use either the standard format (named "xterm+direct" in the
terminfo source) or the legacy format (named "xterm+indirect" in the
terminfo source).

Hence, I believe it makes sense to align the docs with reality.

If you're interested in the story of this escape sequence, I'd recommend
reading
<https://invisible-island.net/ncurses/ncurses.faq.html#xterm_16MegaColors>.

[terminfo]: https://invisible-island.net/ncurses/terminfo.ti.html

closes: #16350

Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Andrea Pappacoda <andrea@pappacoda.it>

Author: Tachi107

runtime/doc/term.txt

@@ -1,4 +1,4 @@
-*term.txt*      For Vim version 9.1.  Last change: 2024 Oct 05
+*term.txt*      For Vim version 9.1.  Last change: 2024 Dec 31
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -688,10 +688,11 @@ The default values are set like this: >
 	 let &t_8f = "\<Esc>[38;2;%lu;%lu;%lum"
 	 let &t_8b = "\<Esc>[48;2;%lu;%lu;%lum"
 
-Some terminals accept the same sequences, but with all semicolons replaced by
-colons (this is actually more compatible, but less widely supported): >
-	 let &t_8f = "\<Esc>[38:2:%lu:%lu:%lum"
-	 let &t_8b = "\<Esc>[48:2:%lu:%lu:%lum"
+Some terminals accept similar sequences, with semicolons replaced by colons
+and an extra colon after the number 2 (this is conformant to the ISO 8613-6
+standard, but less widely supported): >
+	 let &t_8f = "\<Esc>[38:2::%lu:%lu:%lum"
+	 let &t_8b = "\<Esc>[48:2::%lu:%lu:%lum"
 
 These options contain printf strings, with |printf()| (actually, its C
 equivalent hence `l` modifier) invoked with the t_ option value and three