feat(lsp): do completionItem/resolve if completeopt=popup #32820

Problem:
No completionItem/resolve handler.

Solution:
If completeopt=popup is set, invoke completionItem/resolve when
a completion item is selected. Show resolved documentation in popup next
to the completion menu.

runtime/lua/vim/lsp/protocol.lua

@@ -482,6 +482,7 @@ function protocol.make_client_capabilities()
             properties = {
               'additionalTextEdits',
               'command',
+              'documentation',
             },
           },
           tagSupport = {