Add comment to Navigation patch on why its done that way

to avoid accidently changing it in future and breaking things

Spigot-Server-Patches/Optimize-Navigation-Listener.patch

@@ -67,6 +67,9 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 -                // NavigationAbstract navigationabstract = anavigationabstract1[k];
 -                // Spigot end
 +            // Paper start
++            // DO NOT USE AN ITERATOR! This must be a for (;;) to avoid CME.
++            // This is perfectly safe, as additions are ok to be done in this iteration
++            // And Removals are queued instead of immediate.
 +            for (int k = 0; k < this.navigators.size(); ++k) {
 +                NavigationAbstract navigationabstract = this.navigators.get(k);
 +                // Paper end