Document inactive block behaviour (#447)

mesecons_pistons/doc/piston/description.html

@@ -1 +1,3 @@
 Pistons are effectors, they push up to 20 blocks in front of them. The push direction can be set by placing them from different angles.
+A piston pointing into an unloaded block won't extend.
+A piston retracting from an unloaded block works, but gravity-sensitive nodes above the empty space may not fall.