ref(colors): extract help groups

There were help syntax highlights scattered around. THis moves them to
one, dedicated spot.

Additionally, there were some incorrect settings for markdown help
blocks, and they were corrected.

colors/highlite.vim

@@ -295,8 +295,6 @@ local highlight_groups = {
 	Ignore = {fg=gray},
 	Error = {fg=white, bg=red_dark, style='bold'},
 	Todo = {fg=yellow, style={'bold', 'underline'}},
-	helpHyperTextJump = 'Underlined',
-	helpSpecial = 'Function',
 	Hint = {fg=black, bg=magenta, style='bold'},
 	Info = function(self) return {fg=self.Hint.fg, bg=pink_light, style=self.Hint.style} end,
 	Warning = function(self) return {fg=self.Hint.fg, bg=orange, style=self.Hint.style} end,
@@ -550,9 +548,9 @@ local highlight_groups = {
 	mkdCodeEnd = 'mkdCodeStart',
 	mkdHeading = 'Delimiter',
 	mkdItalic  = 'mkdBold',
-	mkdLineBreak = NONE,
+	mkdLineBreak = 'NonText',
 	mkdListItem  = 'Special',
-	mkdRule = 'Underlined',
+	mkdRule = function(self) return {fg=self.Ignore.fg, style={'underline', color=self.Delimiter.fg}} end,
 
 	--[[ 4.3.20. Python ]]
 	pythonBrackets    = 'Delimiter',
@@ -618,10 +616,6 @@ local highlight_groups = {
 	tomlTable = 'Structure',
 
 	--[[ 4.3.27. VimScript ]]
-	helpSpecial  = 'Special',
-	helpHeader   = 'Label',
-	helpHeadline = 'Title',
-	helpSectionDelim = 'Delimiter',
 	vimCmdSep     = 'Delimiter',
 	vimFunction   = 'Function',
 	vimFgBgAttrib = 'Constant',
@@ -712,6 +706,14 @@ local highlight_groups = {
 	coqVernacCmd = 'Statement',
 	coqVernacPunctuation = 'coqTermPunctuation',
 
+	--[[ 4.3.37 Help ]]
+	helpSpecial  = 'Special',
+	helpHeader   = 'Label',
+	helpHeadline = 'Title',
+	helpSectionDelim = 'Delimiter',
+	helpHyperTextJump = 'Underlined',
+	helpSpecial = 'Function',
+
 	--[[ 4.4. Plugins
 		Everything in this section is OPTIONAL. Feel free to remove everything
 		here if you don't want to define it, or add more if there's something