Org-id: Clarify customization documentation for org-id, ts option

* lisp/org-id.el (org-id-ts-format, org-id-method): Clarify docstrings.
* doc/org-manual.org (Handling Links): Mention org-id-method in
the manual.

doc/org-manual.org

@@ -3356,8 +3356,9 @@ current buffer:
   =ID= property for the link[fn:29].  So using this command in Org
   buffers potentially creates two links: a human-readable link from
   the custom ID, and one that is globally unique and works even if the
-  entry is moved from file to file.  Later, when inserting the link,
-  you need to decide which one to use.
+  entry is moved from file to file.  The =ID= property can be either a
+  UUID (default) or a timestamp, depending on ~org-id-method~.  Later,
+  when inserting the link, you need to decide which one to use.
 
 - /Email/News clients: VM, Rmail, Wanderlust, MH-E, Gnus/ ::
 

lisp/org-id.el

@@ -129,8 +129,11 @@ nil   Never use an ID to make a link, instead link using a text search for
   :type 'string)
 
 (defcustom org-id-ts-format "%Y%m%dT%H%M%S.%6N"
-  "Default format for IDs generated using `ts' `org-id-method'.
-The format should be suitable to pass as an argument to `format-time-string'."
+  "Timestamp format for IDs generated using `ts' `org-id-method'.
+The format should be suitable to pass as an argument to `format-time-string'.
+
+Defaults to ISO8601 timestamps without separators and without
+timezone, local time and precision down to 1e-6 seconds."
   :type 'string
   :package-version '(Org . "9.5"))
 
@@ -150,13 +153,12 @@ uuid       Create random (version 4) UUIDs.  If the program defined in
            `org-id-uuid-program' is available it is used to create the ID.
            Otherwise an internal functions is used.
 
-ts         Create ID's based on ISO8601 timestamps (without separators
-           and without timezone, local time).  Precision down to seconds."
+ts         Create ID's based on timestamps as specified in `org-id-ts-format'."
   :group 'org-id
   :type '(choice
 	  (const :tag "Org's internal method" org)
 	  (const :tag "external: uuidgen" uuid)
-	  (const :tag "ISO8601 timestamp" ts)))
+	  (const :tag "Timestamp with format `org-id-ts-format'" ts)))
 
 (defcustom org-id-prefix nil
   "The prefix for IDs.