docs: include NvimTreeWindowPicker in hl-groups (#730)

Author: jlbas

doc/nvim-tree-lua.txt

@@ -567,8 +567,9 @@ HIGHLIGHT GROUPS				        *nvim-tree-highlight*
 
 |nvim_tree_highlight|				        *nvim_tree_highlight*
 
-All the following highlight groups can be configured by hand. It is not
-advised to colorize the background of these groups.
+All the following highlight groups can be configured by hand. Aside from
+`NvimTreeWindowPicker`, it is not advised to colorize the background of these
+groups.
 
 Example (in your `init.vim`):
 >
@@ -619,6 +620,8 @@ NvimTreeGitRenamed
 NvimTreeGitNew
 NvimTreeGitDeleted
 
+NvimTreeWindowPicker
+
 There are also links to normal bindings to style the tree itself.
 
 Normal