org-at-timestamp-p: Fix invalid regexp groups for 'agenda scope

* lisp/org.el (org-at-timestamp-p): Make sure that match groups follow
the docstring in `agenda' scope.  Update docstring explaining return
values for sexp timestamps.

Reported in https://orgmode.org/list/CAKJdtO8bqdbW5N6pRFNQRATjm2daEBDCpJj0njwcd4fmhSJSKQ@mail.gmail.com

lisp/org.el

@@ -15012,20 +15012,24 @@ value is equivalent to `inactive'.
 When at a timestamp, return the position of the point as a symbol
 among `bracket', `after', `year', `month', `hour', `minute',
 `day' or a number of character from the last know part of the
-time stamp.
+time stamp.  If diary sexp timestamps, any point inside the timestamp
+is considered `day' (i.e. only `bracket', `day', and `after' return
+values are possible).
 
 When matching, the match groups are the following:
-  group 1: year
-  group 2: month
-  group 3: day number
-  group 4: day name
+  group 1: year, if any
+  group 2: month, if any
+  group 3: day number, if any
+  group 4: day name, if any
   group 5: hours, if any
   group 6: minutes, if any"
-  (let* ((regexp (if extended
-                     (if (eq extended 'agenda)
-                         org-element--timestamp-regexp
-		       org-ts-regexp3)
-                   org-ts-regexp2))
+  (let* ((regexp
+          (if extended
+              (if (eq extended 'agenda)
+                  (rx (or (regexp org-ts-regexp3)
+                          (regexp org-element--timestamp-regexp)))
+		org-ts-regexp3)
+            org-ts-regexp2))
 	 (pos (point))
 	 (match?
 	  (let ((boundaries (org-in-regexp regexp)))