properly documented SpawnDrops() in GridEntityRock.md (#592)

currently SpawnDrops() is described as a normal function, despite it being static. it is also missing the 1st parameter in the current documentation, which is the position where drops should be spawned.

docs/docs/GridEntityRock.md

@@ -41,4 +41,4 @@ ___
 ### UpdateNeighbors () {: aria-label='Functions' }
 #### void UpdateNeighbors ( ) {: .copyable aria-label='Functions' }
 
-___
+___