runtime(doc): add help tags for json + markdown global variables

I added help tags for them in the syntax.txt file since this is the only
place they are mentioned.

closes: #15486

Signed-off-by: JJCUBER <34446698+JJCUBER@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index f1490c5..8e33d02 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2024 Aug 03
+*syntax.txt*	For Vim version 9.1.  Last change: 2024 Aug 13
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2157,7 +2157,8 @@
 number is that redrawing can become slow.
 
 
-JSON						*json.vim* *ft-json-syntax*
+JSON			*json.vim* *ft-json-syntax* *g:vim_json_conceal*
+					       *g:vim_json_warnings*
 
 The json syntax file provides syntax highlighting with conceal support by
 default. To disable concealment: >
@@ -2347,7 +2348,8 @@
   mv_finance	 mv_logic	mv_powseries
 
 
-MARKDOWN						*ft-markdown-syntax*
+MARKDOWN			*ft-markdown-syntax* *g:markdown_minlines*
+		 *g:markdown_fenced_languages* *g:markdown_syntax_conceal*
 
 If you have long regions there might be wrong highlighting.  At the cost of
 slowing down displaying, you can have the engine look further back to sync on