Disable context menu by default, add documentation.

Users can opt-in to a right-click menu in their ginit.vim file.

Normal Mode:
nnoremap <silent><RightMouse> :call GuiShowContextMenu()<CR>

Insert Mode:
inoremap <silent><RightMouse> <Esc>:call GuiShowContextMenu()<CR>
" <Esc> or <C-O> is required, so the command runs in Normal Mode.
" <Esc> will exit, and <C-O> will remain in Insert Mode.

Visual Modea;
vnoremap <silent><RightMouse> :call GuiShowContextMenu()<CR>gv
" gv causes the selection to persist after the event. The call to
" GuiShowContextMenu wipes the selection, and gv restores it.

src/gui/runtime/doc/nvim_gui_shim.txt

@@ -149,6 +149,14 @@ This can be used to check for a specific GUI in ginit.vim, e.g.
 
 This is a wrapper around |nvim_get_chan_info()|.
 
+							*GuiShowContextMenu()*
+Displays a Cut-Copy-Paste context menu at the current cursor position. This
+menu can be mapped to right click events in ginit.vim, e.g.
+>
+	nnoremap <silent><RightMouse> :call GuiShowContextMenu()<CR>
+	inoremap <silent><RightMouse> <Esc>:call GuiShowContextMenu()<CR>
+	vnoremap <silent><RightMouse> :call GuiShowContextMenu()<CR>gv
+
 ==============================================================================
 4. Internals
 

src/gui/runtime/plugin/nvim_gui_shim.vim

@@ -208,11 +208,6 @@ noremap <script> <Plug>GuiTreeviewToggle :call <SID>TreeViewToggle()
 anoremenu <script> Gui.Treeview.Toggle :call <SID>TreeViewShowToggle()
 
 " Show Right-Click ContextMenu
-function s:GuiShowContextMenu() range
+function GuiShowContextMenu() range
 	call rpcnotify(0, 'Gui', 'ShowContextMenu')
 endfunction
-
-command! -range GuiShowContextMenu call s:GuiShowContextMenu()
-nnoremap <silent><RightMouse> :GuiShowContextMenu<CR>
-inoremap <silent><RightMouse> :GuiShowContextMenu<CR>
-vnoremap <silent><RightMouse> :GuiShowContextMenu<CR>gv