Document atomic-(inc|dec)-fetch, aka ++⚛ and --⚛

Author: lizmat

doc/Type/atomicint.pod6

@@ -137,6 +137,24 @@ too big to unbox to a 64-bit integer, an exception will be thrown. If C<$value>
 otherwise overflows C<atomicint> then it will be silently truncated before the
 subtraction is performed.
 
+=head2 sub atomic-inc-fetch
+
+    multi sub atomic-inc--fetch(atomicint $ is rw)
+
+Performs an atomic increment on a native integer. This will be performed using
+hardware-provided atomic operations. Since the operation is atomic, it is safe
+to use without acquiring a lock. Returns the value resulting from the
+increment. Overflow will wrap around silently.
+
+=head2 sub atomic-dec-fetch
+
+    multi sub atomic-dec--fetch(atomicint $ is rw)
+
+Performs an atomic decrement on a native integer. This will be performed using
+hardware-provided atomic operations. Since the operation is atomic, it is safe
+to use without acquiring a lock. Returns the value resulting from the
+decrement. Overflow will wrap around silently.
+
 =head2 sub cas
 
 Defined as: