Merge pull request #946 from Jnesselr/jnesselr/plannerdoc

Fixed a comment in the planner

Marlin/planner.h

   return(block);
 }
 
-// Gets the current block. Returns NULL if buffer empty
+// Returns true if the buffer has a queued block, false otherwise
 FORCE_INLINE bool blocks_queued() 
 {
   if (block_buffer_head == block_buffer_tail) {