Add a `guile-2' SRFI-0 feature.

* doc/ref/srfi-modules.texi (SRFI-0): Mention the `guile-2' feature.

* module/ice-9/boot-9.scm (%cond-expand-features): Add `guile-2'.

module/ice-9/boot-9.scm

@@ -3258,6 +3258,7 @@ module '(ice-9 q) '(make-q q-length))}."
 (define %cond-expand-features
   ;; Adjust the above comment when changing this.
   '(guile
+    guile-2
     r5rs
     srfi-0   ;; cond-expand itself
     srfi-4   ;; homogenous numeric vectors