Update docstring for zk-id-regexp to provide more guidance

Removing `\\(...\\)` is not strictly needed, but since both `zk-link-regexp` and
`zk-file-name-regexp` add capture groups themselves, it results in a cleaner
regexp.

zk.el

@@ -123,10 +123,14 @@ If you change this value, set `zk-id-regexp' so that
 the zk IDs can be found."
   :type 'string)
 
-(defcustom zk-id-regexp "\\([0-9]\\{12\\}\\)"
+(defcustom zk-id-regexp "[0-9]\\{12\\}"
   "The regular expression used to search for zk IDs.
 Set it so that it matches strings generated with
-`zk-id-format'."
+`zk-id-format'. The expression should not capture any
+explicitly numbered groups.
+
+See `zk-file-name-regexp' and `zk-link-regexp' functions for
+how this regexp is used."
   :type 'regexp)
 
 (defcustom zk-tag-regexp "\\s#[a-zA-Z0-9]\\+"