bug#26182: cond-expand doc omits guile-2.2 feature

[Zefram]

In Guile 2.2.0, the SRFI-0 (cond-expand) documentation says:

! The Guile core has the following features,
!
!      guile
!      guile-2  ;; starting from Guile 2.x
!      r5rs
!      srfi-0
...

As implemented in Guile 2.2.0, the unlisted feature guile-2.2 is also
recognised by cond-expand.  Since the documentation's list is otherwise
complete, presumably it is intended to be a complete list, and the
omission of this feature from the list is a mistake.  In any case,
it would be helpful to list it.

-zefram