doc: Document `C-c C-x C-j' behavior in the agenda buffer.

In the agenda buffer, `C-c C-x C-j' jumps to the headline of the
currently clocked in task.

doc/org.texi

@@ -5599,9 +5599,10 @@ Cancel the current clock.  This is useful if a clock was started by
 mistake, or if you ended up working on something else.
 @kindex C-c C-x C-j
 @item C-c C-x C-j
-Jump to the entry that contains the currently running clock.  With a
-@kbd{C-u} prefix arg, select the target task from a list of recently clocked
-tasks.
+In the agenda buffer, jump to the headline of the currently clocked in task.
+In an org file, jump to the entry that contains the currently running clock.
+With a @kbd{C-u} prefix arg, select the target task from a list of recently
+clocked tasks.
 @kindex C-c C-x C-d
 @item C-c C-x C-d
 @vindex org-remove-highlights-with-change