docs: apply current colorscheme to default highlight groups

Problem: Not all default highlight groups show their actual colors.
Solution: Refactor `vimhelp.lua` and apply it to all relevant lists (UI
groups, syntax groups, treesitter groups, LSP groups, diagnostic groups).

runtime/doc/lsp.txt

@@ -480,7 +480,7 @@ Use |LspTokenUpdate| and |vim.lsp.semantic_tokens.highlight_token()| for more
 complex highlighting.
 
 The following groups are linked by default to standard |group-name|s:
->
+
 @lsp.type.class          Structure
 @lsp.type.decorator      Function
 @lsp.type.enum           Structure
@@ -496,7 +496,7 @@ The following groups are linked by default to standard |group-name|s:
 @lsp.type.type           Type
 @lsp.type.typeParameter  TypeDef
 @lsp.type.variable       Identifier
-<
+
 ==============================================================================
 EVENTS                                                            *lsp-events*
 

runtime/doc/syntax.txt

@@ -225,7 +225,7 @@ you can see the actual color, except for "Ignore"):
 Type		int, long, char, etc.
 StorageClass	static, register, volatile, etc.
 Structure	struct, union, enum, etc.
-	 Typedef	A typedef
+Typedef		a typedef
 
 Special		any special symbol
 SpecialChar	special character in a constant

runtime/doc/treesitter.txt

@@ -393,7 +393,7 @@ instance, to highlight comments differently per language: >vim
     hi link @comment.doc.java String
 <
 The following captures are linked by default to standard |group-name|s:
->
+
 @text.literal       Comment
 @text.reference     Identifier
 @text.title         Title
@@ -444,7 +444,7 @@ The following captures are linked by default to standard |group-name|s:
 @preproc            PreProc
 @debug              Debug
 @tag                Tag
-<
+
                                                   *treesitter-highlight-spell*
 The special `@spell` capture can be used to indicate that a node should be
 spell checked by Nvim's builtin |spell| checker. For example, the following

runtime/ftplugin/help.lua

@@ -2,6 +2,27 @@
 vim.treesitter.start()
 
 -- add custom highlights for list in `:h highlight-groups`
-if vim.endswith(vim.fs.normalize(vim.api.nvim_buf_get_name(0)), '/doc/syntax.txt') then
-  require('vim.vimhelp').highlight_groups()
+local bufname = vim.fs.normalize(vim.api.nvim_buf_get_name(0))
+if vim.endswith(bufname, '/doc/syntax.txt') then
+  require('vim.vimhelp').highlight_groups({
+    { start = [[\*group-name\*]], stop = '^======', match = '^(%w+)\t' },
+    { start = [[\*highlight-groups\*]], stop = '^======', match = '^(%w+)\t' },
+  })
+elseif vim.endswith(bufname, '/doc/treesitter.txt') then
+  require('vim.vimhelp').highlight_groups({
+    {
+      start = [[\*treesitter-highlight-groups\*]],
+      stop = [[\*treesitter-highlight-spell\*]],
+      match = '^@[%w%p]+',
+    },
+  })
+elseif vim.endswith(bufname, '/doc/diagnostic.txt') then
+  require('vim.vimhelp').highlight_groups({
+    { start = [[\*diagnostic-highlights\*]], stop = '^======', match = '^(%w+)' },
+  })
+elseif vim.endswith(bufname, '/doc/lsp.txt') then
+  require('vim.vimhelp').highlight_groups({
+    { start = [[\*lsp-highlight\*]], stop = '^------', match = '^(%w+)' },
+    { start = [[\*lsp-semantic-highlight\*]], stop = '^======', match = '^@[%w%p]+' },
+  })
 end

runtime/lua/vim/vimhelp.lua

@@ -2,28 +2,30 @@
 
 local M = {}
 
--- Called when editing the doc/syntax.txt file
-function M.highlight_groups()
-  local save_cursor = vim.fn.getcurpos()
-
-  local start_lnum = vim.fn.search([[\*highlight-groups\*]], 'c')
-  if start_lnum == 0 then
-    return
-  end
-  local end_lnum = vim.fn.search('^======')
-  if end_lnum == 0 then
-    return
-  end
-
+--- Apply current colorscheme to lists of default highlight groups
+---
+--- Note: {patterns} is assumed to be sorted by occurrence in the file.
+--- @param patterns {start:string,stop:string,match:string}[]
+function M.highlight_groups(patterns)
   local ns = vim.api.nvim_create_namespace('vimhelp')
   vim.api.nvim_buf_clear_namespace(0, ns, 0, -1)
 
+  local save_cursor = vim.fn.getcurpos()
+
+  for _, pat in pairs(patterns) do
+    local start_lnum = vim.fn.search(pat.start, 'c')
+    local end_lnum = vim.fn.search(pat.stop)
+    if start_lnum == 0 or end_lnum == 0 then
+      break
+    end
+
     for lnum = start_lnum, end_lnum do
-    local word = vim.api.nvim_buf_get_lines(0, lnum - 1, lnum, true)[1]:match('^(%w+)\t')
+      local word = vim.api.nvim_buf_get_lines(0, lnum - 1, lnum, true)[1]:match(pat.match)
       if vim.fn.hlexists(word) ~= 0 then
         vim.api.nvim_buf_set_extmark(0, ns, lnum - 1, 0, { end_col = #word, hl_group = word })
       end
     end
+  end
 
   vim.fn.setpos('.', save_cursor)
 end