org-capture: Document org-directory

* lisp/org-capture.el (org-capture-templates):
* doc/org.texi (Template elements): Document org-directory.

doc/org.texi

@@ -7154,7 +7154,9 @@ files, targets usually define a node.  Entries will become children of this
 node.  Other types will be added to the table or list in the body of this
 node.  Most target specifications contain a file name.  If that file name is
 the empty string, it defaults to @code{org-default-notes-file}.  A file can
-also be given as a variable, function, or Emacs Lisp form.
+also be given as a variable, function, or Emacs Lisp form.  When an absolute
+path is not specified for a target, it is taken as relative to
+@code{org-directory}.
 
 Valid values are:
 

lisp/org-capture.el

@@ -124,7 +124,8 @@ target       Specification of where the captured item should be placed.
              Most target specifications contain a file name.  If that file
              name is the empty string, it defaults to `org-default-notes-file'.
              A file can also be given as a variable, function, or Emacs Lisp
-             form.
+             form.  When an absolute path is not specified for a
+             target, it is taken as relative to `org-directory'.
 
              Valid values are: