add note about macOS share and exec policy

stdlib/Mmap/src/Mmap.jl

@@ -166,6 +166,10 @@ will be visible to other processes mapping the same file.
 
 The `exec` keyword argument specifies whether the underlying mmap data will be executable.
 
+!!! note
+    On MacOS `exec=true` implies `shared=false`, because each thread has its own access permissions to `mmap` regions. 
+
+
 For example, the following code
 
 ```julia