Add 'eldoc-box-help-at-point'

* eldoc-box.el (eldoc-box--help-at-point-last-point): New variable
(eldoc-box--help-at-point-cleanup): New function.
(eldoc-box-help-at-point): New command.

eldoc-box.el

@@ -178,6 +178,32 @@ Intended for internal use."
     (delete-frame eldoc-box--frame)
     (setq eldoc-box--frame nil)))
 
+;;;;; Help at point
+
+(defvar eldoc-box--help-at-point-last-point 0
+  "This point cache is used by clean up function.
+If (point) != last point, cleanup frame.")
+
+(defun eldoc-box--help-at-point-cleanup ()
+  "Try to clean up the childframe made by eldoc-box hack."
+  (if (or (eq (point) eldoc-box--help-at-point-last-point)
+          ;; don't clean up when the user clicks childframe
+          (eq (selected-frame) eldoc-box--frame))
+      (run-with-timer 0.1 nil #'eldoc-box--help-at-point-cleanup)
+    (eldoc-box-quit-frame)
+    (kill-local-variable 'eldoc-display-functions)))
+
+(defun eldoc-box-help-at-point ()
+  "Display documentation of the symbol at point."
+  (interactive)
+  (let ((eldoc-box-position-function
+         #'eldoc-box--default-at-point-position-function))
+    (eldoc-box--display
+     (with-current-buffer eldoc--doc-buffer
+       (buffer-string))))
+  (setq eldoc-box--help-at-point-last-point (point))
+  (run-with-timer 0.1 nil #'eldoc-box--help-at-point-cleanup))
+
 ;;;; Backstage
 ;;;;; Variable
 (defvar eldoc-box--buffer " *eldoc-box*"