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.

mesecons_pistons/doc/piston_sticky/description.html

@@ -1 +1,4 @@
 Sticky 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. Sticky ones also pull 1 block.
+A sticky piston pointing into an unloaded block won't extend.
+A sticky piston retracting from within an unloaded block works and pulls a node, but if it doesn't pull anything, then gravity-sensitive nodes above the empty space may not fall.
+An extended sticky piston that touches the surface of an unloaded block and loses signal retracts but doesn't pull anything.