Improvements to apropos and describe-symbol

Patch the apropos button types so they call helpful instead of the
built-in describe functions.  Also add some bindings to apropos-mode-map
so it behaves like other help modes.

Add `doom/describe-symbol` function, which shows documentation for
callable and variable symbols.  If a symbol is both a variable and a
callable, it dispatches to apropos.  This gives a better workflow than
`helpful-symbol`, which annoyingly prompts the user.

Remap `describe-symbol` to `doom/describe-symbol`, and update
`+emacs-lisp-lookup-documentation` to call it also.

modules/config/default/+emacs-bindings.el

@@ -320,6 +320,11 @@
       (:after helpful-mode
         :map helpful-mode-map
         "o" #'ace-link-help)
+      (:after apropos
+        :map apropos-mode-map
+        "o" #'ace-link-help
+        "n" #'forward-button
+        "p" #'backward-button)
       (:after info
         :map Info-mode-map
         "o" #'ace-link-info)