GH-116218 Docs: Add availability information for the 'resource' module (#116256)

Add availability data to 'resource' module Docs
diff --git a/Doc/library/resource.rst b/Doc/library/resource.rst
index 4e58b04..389a63f 100644
--- a/Doc/library/resource.rst
+++ b/Doc/library/resource.rst
@@ -177,6 +177,8 @@
 
    The largest area of mapped memory which the process may occupy.
 
+   .. availability:: FreeBSD >= 11.
+
 
 .. data:: RLIMIT_AS