runtime: Enhance help.vim to highlight Nvim/NVIM

runtime/syntax/help.vim

@@ -42,6 +42,8 @@ syn match helpNormal		"|||"
 syn match helpNormal		":|vim:|"	" for :help modeline
 syn match helpVim		"\<Vim version [0-9][0-9.a-z]*"
 syn match helpVim		"VIM REFERENCE.*"
+syn match helpVim		"\<Nvim\."
+syn match helpVim		"NVIM REFERENCE.*"
 syn match helpOption		"'[a-z]\{2,\}'"
 syn match helpOption		"'t_..'"
 syn match helpCommand		"`[^` \t]\+`"hs=s+1,he=e-1 contains=helpBacktick