Specs: Define when needsLayout is reset (also, some unrelated markdown fixes)

sky/examples/style/block-layout.sky

 SKY MODULE
 <import src="sky:core" as="sky"/>
 <!--
  ! this module provides trivial vertical block layout
  ! no margins, padding, borders, etc
  !-->
 <script>
  module.exports.BlockLayoutManager = class BlockLayoutManager extends sky.LayoutManager {
    function layout(width, height) {
+     this.markAsLaidOut();
      if (width == null)
        width = this.getIntrinsicWidth().value;
      let autoHeight = false;
      if (height == null) {
        height = 0;
        autoHeight = true;
      }
      this.assumeDimensions(width, height);
      let children = this.walkChildren();
      let loop = children.next();
@@ skipping 40 matching lines @@
        let children = this.walkChildren();
        let loop = children.next();
        while (!loop.done) {
          let child = loop.value;
          let childHeight = child.layoutManager.getIntrinsicHeight();
          if (height < childHeight.value)
            height = childHeight.value;
          loop = children.next();
        }
      }
-     return super(height); // applies and provides our own min-width/max-width rules
+     return super(height); // applies and provides our own min-height/max-height rules
    }
  }
  sky.registerLayoutManager('block', module.exports.BlockLayoutManager);
 </script>