Add racket-eldoc-app-full-docs

Although I think it's too slow to be enabled by default, let's see.

Note: It's not quite as bad when the racket-scribble.el file is native
compiled... but in my experience, it still wasn't great.

To use in a racket-mode buffer that also has racket-xp-mode enabled:

  (remove-hook 'eldoc-documentation-functions
               #'racket-eldoc-app
               t)

  (add-hook 'eldoc-documentation-functions
            #'racket-eldoc-app-full-docs
            nil t)

racket-eldoc.el

@@ -9,6 +9,7 @@
 ;; SPDX-License-Identifier: GPL-3.0-or-later
 
 (require 'racket-cmd)
+(require 'racket-company-doc)
 
 ;; Given that `eldoc-documentation-functions' is plural, you might
 ;; wonder why `racket--eldoc-sexp-app' employs two methods. Wouldn't
@@ -70,6 +71,29 @@ the surface syntax, or Typed Racket type information."
          (racket--eldoc-do-callback callback thing str)))
       t)))
 
+(defun racket-eldoc-app-full-docs (callback &rest _more)
+  "Call eldoc CALLBACK with full docs about app around point.
+Assume sexp language. Use doc text property added by racket-xp-mode.
+Can be slow when the doc HTML is large/complicated, so, probably
+won't enable this by default."
+  (when (and (racket--cmd-open-p)
+             (> (point) (point-min)))
+    (save-excursion
+      (ignore-errors
+        (backward-up-list)
+        (forward-char 1)
+        (pcase (get-text-property (point) 'racket-xp-doc)
+          (`(,path ,anchor ,_tag)
+           (let* ((end (next-single-property-change (point) 'racket-xp-doc))
+                  (thing (buffer-substring-no-properties (point) end)))
+             (racket--eldoc-do-callback
+              callback
+              thing
+              (with-temp-buffer
+                (racket--scribble-path+anchor-insert path anchor)
+                (buffer-string)))
+             t)))))))
+
 (defun racket--eldoc-do-callback (callback thing str)
   (if str
       (funcall callback