patch 9.1.1138: cmdline completion for :hi is too simplistic

Problem:  Existing cmdline completion for :highlight was barebone and
          only completed the highlight group names.

Solution: Implement full completion for the highlight group arguments
          such as guifg and cterm. If the user tries to complete
          immediately after the '=' (e.g. `hi Normal guifg=<Tab>`), the
          completion will fill in the existing value, similar to how
          cmdline completion for options work (Yee Cheng Chin).

closes: #16712

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index cb6704c..d8c4b08 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2025 Feb 20
+*syntax.txt*	For Vim version 9.1.  Last change: 2025 Feb 23
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -5432,6 +5432,10 @@
 			See |:highlight-default| for the optional [default]
 			argument.
 
+:hi[ghlight][!] [default] link {from-group} {to-group}
+:hi[ghlight][!] [default] link {from-group} NONE
+			See |:hi-link|.
+
 Normally a highlight group is added once when starting up.  This sets the
 default values for the highlighting.  After that, you can use additional
 highlight commands to change the arguments that you want to set to non-default