docs: add hl-SnippetTabstop tag

runtime/doc/lua.txt

@@ -4338,7 +4338,7 @@ vim.snippet.expand({input})                             *vim.snippet.expand()*
     https://microsoft.github.io/language-server-protocol/specification/#snippet_syntax
     for the specification of valid input.
 
-    Tabstops are highlighted with hl-SnippetTabstop.
+    Tabstops are highlighted with |hl-SnippetTabstop|.
 
     Parameters: ~
       • {input}  (`string`)

runtime/doc/syntax.txt

@@ -5054,6 +5054,8 @@ QuickFixLine	Current |quickfix| item in the quickfix window. Combined with
 							*hl-Search*
 Search		Last search pattern highlighting (see 'hlsearch').
 		Also used for similar items that need to stand out.
+							*hl-SnippetTabstop*
+SnippetTabstop	Tabstops in snippets. |vim.snippet|
 							*hl-SpecialKey*
 SpecialKey	Unprintable characters: Text displayed differently from what
 		it really is. But not 'listchars' whitespace. |hl-Whitespace|

runtime/lua/vim/snippet.lua

@@ -401,7 +401,7 @@ end
 --- Refer to https://microsoft.github.io/language-server-protocol/specification/#snippet_syntax
 --- for the specification of valid input.
 ---
---- Tabstops are highlighted with hl-SnippetTabstop.
+--- Tabstops are highlighted with |hl-SnippetTabstop|.
 ---
 --- @param input string
 function M.expand(input)