document fdManager change

documentation/RELEASE_NOTES.md

@@ -22,6 +22,12 @@ should also be read to understand what has changed since earlier releases:
 
 ## Changes made on the 7.0 branch since 7.0.8.1
 
+### fdManager file descriptor limit removed
+
+In order to support file descriptors above 1023, fdManager now uses
+poll() instead of select() on all architectures that support it
+(Linux, MacOS, Windows, newer RTEMS).
+
 ### Post monitors from compress record when it's reset
 
 Writing into a compress record's `RES` field now posts a monitor event instead