Add comments explaining the context state

Author: kitten

src/internals/context.js

@@ -7,6 +7,16 @@ import type {
   ContextStore
 } from '../types'
 
+/* The context is kept as a Map from a Context value to the current
+   value on the React element tree.
+   The legacy context is kept as a simple object.
+   When the tree is being walked modifications are made by assigning
+   new legacy context maps or new context values.
+   These changes are kept in the `prev` variables and must be flushed
+   before continuing to walk the tree.
+   After walking the children they can be restored.
+   This way the context recursively restores itself on the way up. */
+
 type ContextEntry = [AbstractContext, mixed]
 
 let currentContextStore: ContextStore = new Map()

src/visitor.js

@@ -152,11 +152,13 @@ const visitChild = (
   visitor: Visitor
 ) => {
   const children = visitElement(child, queue, visitor)
+  // Flush the context changes
   const prevMap = flushPrevContextMap()
   const prevStore = flushPrevContextStore()
 
   visitChildren(children, queue, visitor)
 
+  // Restore context changes after children have been walked
   if (prevMap !== undefined) {
     restoreContextMap(prevMap)
   }