Add additional info to lsp-ui-doc-enable docs (#669)

I initially didn't expect lsp-ui-doc-enable to only be read when the
buffer was started, I thought it would take effect for buffers that were
already running.

Author: axelson

lsp-ui-doc.el

@@ -63,7 +63,8 @@
   :link '(info-link "(lsp-ui-doc) Customizing"))
 
 (defcustom lsp-ui-doc-enable t
-  "Whether or not to enable lsp-ui-doc."
+  "Whether or not to enable lsp-ui-doc.
+Displays documentation of the symbol at point on hover. This only takes effect when a buffer is started."
   :type 'boolean
   :group 'lsp-ui)