[gjs: 1/2] doc: note the version `constructor()` became supported

[Philip Chimento <pchimento src gnome org>]

commit cdfeb12aad7c42f0fc50447eede60e2510353141
Author: Andy Holmes <andrew g r holmes gmail com>
Date:   Wed Jul 20 20:02:56 2022 -0700

    doc: note the version `constructor()` became supported
    
    Since older versions of GNOME continue to hang around for a while, note
    the previous "workaround" for `constructor()`.

 doc/Mapping.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/doc/Mapping.md b/doc/Mapping.md
index 130145386..e635a6674 100644
--- a/doc/Mapping.md
+++ b/doc/Mapping.md
@@ -48,6 +48,8 @@ var MyLabel = GObject.registerClass({
 });
 ```
 
+Note that before GJS 1.72 (GNOME 42), you had to override `_init()` instead of `constructor()` and chain-up 
with `super._init()`. This is still supported in GJS 1.72 and later.
+
 ### GType Objects
 
 This is the object that represents a type in the GObject type system. Internally a GType is an integer, but 
you can't access that integer in GJS.