doc: Update 'skeletons' field documentation.

* doc/guix.texi (operating-system Reference): Update documentation of
'skeletons' as a followup to e79467f63a.

doc/guix.texi

@@ -6426,15 +6426,17 @@ For example, @code{'("/dev/sda3")}.
 List of user accounts and groups.  @xref{User Accounts}.
 
 @item @code{skeletons} (default: @code{(default-skeletons)})
-A monadic list of pairs of target file name and files.  These are the
-files that will be used as skeletons as new accounts are created.
+A list target file name/file-like object tuples (@pxref{G-Expressions,
+file-like objects}).  These are the skeleton files that will be added to
+the home directory of newly-created user accounts.
 
 For instance, a valid value may look like this:
 
 @example
-(mlet %store-monad ((bashrc (text-file "bashrc" "\
-     export PATH=$HOME/.guix-profile/bin")))
-  (return `((".bashrc" ,bashrc))))
+`((".bashrc" ,(plain-file "bashrc" "echo Hello\n"))
+  (".guile" ,(plain-file "guile"
+                         "(use-modules (ice-9 readline))
+                          (activate-readline)")))
 @end example
 
 @item @code{issue} (default: @var{%default-issue})