Use CSS terminology for sizing rules (#1460)

Summary:
X-link: https://github.com/facebook/react-native/pull/41390

Pull Request resolved: https://github.com/facebook/yoga/pull/1460

Yoga passes `MeasureMode`/`YGMeasureMode` to express constraints in how a box should be measured, given definite or indefinite available space.

This is modeled after Android [MeasureSpec](https://developer.android.com/reference/android/view/View.MeasureSpec), with a table above `calculateLayoutImpl()` explaining the CSS terms they map to. This can be confusing when flipping between the spec, and code.

This switches internal usages to the CSS terms, but leaves around `YGMeasureMode` since it is the public API passed to measure functions.

Reviewed By: joevilches

Differential Revision: D51068417

fbshipit-source-id: 0a76266a4e7e0cc39996164607229c3c41de2818

yoga/YGNode.h

@@ -186,6 +186,19 @@ typedef struct YGSize {
 } YGSize;
 
 /**
+ * Returns the computed dimensions of the node, following the contraints of
+ * `widthMode` and `heightMode`:
+ *
+ * YGMeasureModeUndefined: The parent has not imposed any constraint on the
+ * child. It can be whatever size it wants.
+ *
+ * YGMeasureModeAtMost: The child can be as large as it wants up to the
+ * specified size.
+ *
+ * YGMeasureModeExactly: The parent has determined an exact size for the
+ * child. The child is going to be given those bounds regardless of how big it
+ * wants to be.
+ *
  * @returns the size of the leaf node, measured under the given contraints.
  */
 typedef YGSize (*YGMeasureFunc)(