Document customization of popup window color (#415)

* Allow customizing popup window highlight group

* Add documentation

* Revert "Add documentation"

This reverts commit 41954eb.

* Revert "Allow customizing popup window highlight group"

This reverts commit 625439b.

* Add customization instructions to documentation

Author: thomasfaingnaert

doc/vim-lsp.txt

@@ -170,24 +170,50 @@ g:lsp_preview_float                         *g:lsp_preview_float*
     are shown in a floating window as |preview-window| at the cursor position.
     The |preview-window| is closed automatically on cursor moves, unless it is
     focused. While focused it may be closed with <esc>.
-    After opening an autocmd User event lsp_float_opened is issued, as well as
-    and lsp_float_closed upon closing. This can be used to alter the preview
-    window (using lsp#ui#vim#output#getpreviewwinid() to get the window id), or
-    setup custom bindings while a preview is open.
+
     This feature requires neovim 0.4.0 (current master) or
     Vim8.1 with has('patch-8.1.1517').
 
     Example:
+    >
 	" Opens preview windows as floating
 	let g:lsp_preview_float = 1
 
 	" Opens preview windows as normal windows
 	let g:lsp_preview_float = 0
+    <
 
+    After opening an autocmd User event lsp_float_opened is issued, as well as
+    and lsp_float_closed upon closing. This can be used to alter the preview
+    window (using lsp#ui#vim#output#getpreviewwinid() to get the window id),
+    setup custom bindings while a preview is open, or change the highlighting
+    of the window.
+
+    Example of custom keybindings:
+    >
 	" Close preview window with <esc>
 	autocmd User lsp_float_opened nmap <buffer> <silent> <esc>
 		      \ <Plug>(lsp-preview-close)
 	autocmd User lsp_float_closed nunmap <buffer> <esc>
+    <
+
+    Example of customising the highlighting:
+    >
+	highlight PopupWindow ctermbg=lightblue guibg=lightblue
+
+	augroup lsp_float_colours
+	    autocmd!
+	    if !has('nvim')
+		autocmd User lsp_float_opened
+		    \ call win_execute(lsp#ui#vim#output#getpreviewwinid(),
+		    \		       'setlocal wincolor=PopupWindow')
+	    else
+		autocmd User lsp_float_opened
+		    \ call nvim_win_set_option(lsp#ui#vim#output#getpreviewwinid(),
+		    \			       'winhighlight', 'Normal:PopupWindow')
+	    endif
+	augroup end
+    <
 
 g:lsp_preview_autoclose                         *g:lsp_preview_autoclose*
     Type: |Number|
@@ -750,7 +776,7 @@ Closes an opened preview window
 Transfers focus to an opened preview window or back to the previous window if
 focus is already on the preview window.
 
-  
+
 ===============================================================================
 Autocomplete                                          *vim-lsp-autocomplete*