Rephrase deprecation messages for clarity

**what is the change?:**
We rephrased the deprecation messages to clarify that
 - these APIs are currently deprecated
 - they will be removed in React v16.0

The previous wording implied that they would be deprecated in v16.0.

**why make this change?:**
To make the messages easier to understand.

**test plan:**
Visual inspection

src/isomorphic/React.js

@@ -59,9 +59,10 @@ if (__DEV__) {
   createMixin = function(mixin) {
     lowPriorityWarning(
       warnedForCreateMixin,
-      'In React 16.0, React.createMixin is deprecated and should not be used. ' +
-        'You can use this mixin directly instead.' +
-        ' See (https://fb.me/createmixin-was-never-implemented) for more info.',
+      'React.createMixin is deprecated and should not be used. ' +
+        'In React v16.0, it will be removed. ' +
+        'You can use this mixin directly instead. ' +
+        'See https://fb.me/createmixin-was-never-implemented for more info.',
     );
     warnedForCreateMixin = true;
     return mixin;
@@ -110,8 +111,9 @@ if (__DEV__) {
       get() {
         lowPriorityWarning(
           didWarnPropTypesDeprecated,
-          'In React v16.0+, accessing PropTypes via the main React package is deprecated. Use ' +
-            'the prop-types package from npm instead.' +
+          'Accessing PropTypes via the main React package is deprecated,' +
+            ' and will be removed in  React v16.0.' +
+            ' Use the prop-types package from npm instead.' +
             ' Version 15.5.10 provides a drop-in replacement.' +
             ' For info on usage, compatibility, migration and more, see ' +
             'https://fb.me/prop-types-docs',