Explicitly state the signal update is AMO

Based on recent review comments, it looks like it would be more
clear if we state that the signal update is an atomic operation.
We have added this as part of the Notes to Implementers section.

content/shmem_put_signal.tex

@@ -77,6 +77,13 @@
     routine does not imply delivery of the put routine's data.
 }
 
+\apiimpnotes{
+    Implementations must ensure that put-with-signal routines are compatible
+    with all point-to-point synchronization interfaces. The delivery of
+    \signal{} flag on the remote \ac{PE} must not cause partial updates. This
+    requires the update on \signal{} flag to be an atomic memory operation.
+}
+
 \begin{apiexamples}
 
 \apicexample