docs: add highlight examples to quickstart, document pre-overhaul SpellCap groups (#2680)

* docs: add highlight examples to quickstart, document pre-overhaul SpellLocal groups

* docs: add highlight examples to quickstart

* docs: document pre-overhaul SpellCap groups

* docs: document pre-overhaul SpellCap groups

Author: alex-courtis

README.md

@@ -112,6 +112,23 @@ require("nvim-tree").setup {
 }
 ```
 
+### Highlight
+
+Run `:NvimTreeHiTest` to show all the highlights that nvim-tree uses.
+
+They can be customised before or after setup is called and will be immediately
+applied at runtime. e.g.
+
+```lua
+vim.cmd([[
+    :hi      NvimTreeExecFile    guifg=#ffa0a0
+    :hi      NvimTreeSpecialFile guifg=#ff80ff gui=underline
+    :hi      NvimTreeSymlink     guifg=Yellow  gui=italic
+    :hi link NvimTreeImageFile   Title
+]])
+```
+See [:help nvim-tree-highlight](doc/nvim-tree-lua.txt) for details.
+
 ## Commands
 
 See [:help nvim-tree-commands](doc/nvim-tree-lua.txt)

doc/nvim-tree-lua.txt

@@ -238,8 +238,15 @@ via |nvim-tree.on_attach| e.g. >
 Run |:NvimTreeHiTest| to show all the highlights that nvim-tree uses.
 
 They can be customised before or after setup is called and will be immediately
-applied at runtime.
-
+applied at runtime. e.g. >
+
+    vim.cmd([[
+        :hi      NvimTreeExecFile    guifg=#ffa0a0
+        :hi      NvimTreeSpecialFile guifg=#ff80ff gui=underline
+        :hi      NvimTreeSymlink     guifg=Yellow  gui=italic
+        :hi link NvimTreeImageFile   Title
+    ]])
+<
 See |nvim-tree-highlight| for details.
 
 ==============================================================================
@@ -2444,6 +2451,15 @@ Diagnostics Folder Highlight: >
 - NvimTreeSpecialFile  PreProc   -> SpellCap
 - NvimTreeSymlink      Statement -> SpellCap
 
+Approximate pre-overhaul values for the `SpellCap` groups may be set via: >
+
+    vim.cmd([[
+        :hi NvimTreeExecFile    gui=bold           guifg=#ffa0a0
+        :hi NvimTreeSymlink     gui=bold           guifg=#ffff60
+        :hi NvimTreeSpecialFile gui=bold,underline guifg=#ff80ff
+        :hi NvimTreeImageFile   gui=bold           guifg=#ff80ff
+    ]])
+<
 Legacy highlight group are still obeyed when they are defined and the current
 highlight group is not, hard linking as follows: >