[guile] Update supported guile versions in docs

Document that Guile 2.2.x and 3.0.x work.

Document that Guile 1.8.x is believed to still work, but that we're
no longer able to easily test this.

Doc/Manual/Guile.html

@@ -52,9 +52,10 @@ <H2><a name="Guile_nn1">26.1 Supported Guile Versions</a></H2>
 
 
 <p>
-SWIG works with Guile versions 1.8.x and 2.0.x. Support for version
-1.6.x has been dropped. The last version of SWIG that still works with
-Guile version 1.6.x is SWIG 2.0.9.
+SWIG is known to work with Guile versions 2.0.x, 2.2.x and 3.0.x. We believe
+it still works with 1.8.x, but we're no longer able to easily test this.
+Support for version 1.6.x has been dropped. The last version of SWIG that still
+works with Guile version 1.6.x is SWIG 2.0.9.
 
 <p>
 Note that starting with guile 2.0, the guile sources can be compiled for