Improve posframe signature help UI (#2638)

* Add border to posframe

* Change border to background color to look like padding

Author: ericdallo

lsp-mode.el

@@ -4946,10 +4946,13 @@ RENDER-ALL - nil if only the signature should be rendered."
         :background-color (face-attribute 'tooltip :background)
         :height 6
         :width 60
+        :border-width 10
+        :border-color (face-attribute 'tooltip :background)
         :min-width 60)
   "Params for signature and `posframe-show'.")
 
 (defun lsp-signature-posframe (str)
+  "Use posframe to show the STR signatureHelp string."
   (if str
       (apply #'posframe-show
              (with-current-buffer (get-buffer-create "*lsp-signature*")