SDL3/SDL_CopyProperties: clarify introduction of deadlock risk

Live page is here: https://wiki.libsdl.org/SDL3/SDL_CopyProperties

Author: stefani-linguisticplasticity

SDL3/SDL_CopyProperties.md

@@ -34,7 +34,8 @@ overwritten.
 
 ## Thread Safety
 
-It is safe to call this function from any thread.
+It is safe to call this function from any thread.  This function acquires
+simultaneous mutex locks on both the source and destination property sets.
 
 ## Version