gnu: gccgo: Add note about provided go version.

* gnu/packages/gcc.scm (gccgo-10, gccgo-11, gccgo-12): Add notes about
what version of is provided by these packages.

Change-Id: Id483319e606fdebe1820abeb7e129c580bf2c165

gnu/packages/gcc.scm

@@ -1223,12 +1223,15 @@ provides the GNU compiler for the Go programming language."))
               ;; a cyclic dependency.  <http://debbugs.gnu.org/18101>
               #:separate-lib-output? #f))
 
+;; Provides go-1.14.6
 (define-public gccgo-10
   (make-gccgo gcc-10))
 
+;; Provides go-1.16.5
 (define-public gccgo-11
   (make-gccgo gcc-11))
 
+;; Provides go-1.18
 (define-public gccgo-12
   (make-gccgo gcc-12))