Remove trailing whitespace from `lsp-describe-thing-at-point' (#4576)

When calling `lsp-describe-thing-at-point' trailing whitespace can leak
into the help buffer. Depending on the face settings, this whitespace
can be visibile.

Instead of removing trailing whitespace at the end of the buffer, do so
on a line-by-line basis.

Fixes #4572

lsp-mode.el

@@ -5409,7 +5409,10 @@ If EXCLUDE-DECLARATION is non-nil, request the server to include declarations."
             (delay-mode-hooks
               (lsp-help-mode)
               (with-help-window lsp-help-buf-name
-                (insert (string-trim-right (lsp--render-on-hover-content contents t)))))
+                (insert
+		 (mapconcat 'string-trim-right
+			    (split-string (lsp--render-on-hover-content contents t) "\n")
+			    "\n"))))
             (run-mode-hooks)))
       (lsp--info "No content at point."))))