States when they were introduced.

Refs #2150. I'm not sure if this is it, so not closing it for now.

doc/Type/atomicint.pod6

@@ -7,9 +7,12 @@
     class atomicint is Int is repr('P6int') { }
 
 An C<atomicint> is a native integer sized such that CPU-provided atomic
-operations can be performed upon it. On a 32-bit CPU it will typically be
-32 bits in size, and on an a 64-bit CPU it will typically be 64 bits in size.
-It exists to allow writing portable code that uses atomic operations.
+operations can be performed upon it. On a 32-bit CPU it will typically
+be 32 bits in size, and on an a 64-bit CPU it will typically be 64 bits
+in size. It exists to allow writing portable code that uses atomic
+operations.
+
+Please note that C<atomicint>s were introduced in Rakudo 2017.08. 
 
     # Would typically only work on a 64-bit machine and VM build.
     my int64 $active = 0;