Document that context is not reactive. (#3296)

Author: pngwn

site/content/docs/03-run-time.md

@@ -156,6 +156,8 @@ Like lifecycle functions, this must be called during component initialisation.
 </script>
 ```
 
+> Context is not inherently reactive. If you need reactive values in context then you can pass a store into context, which *will* be reactive.
+
 #### `getContext`
 
 ```js

site/content/tutorial/15-context/01-context-api/text.md

@@ -44,4 +44,6 @@ In `mapbox.js` you'll see this line:
 const key = {};
 ```
 
-We can use anything as a key — we could do `setContext('mapbox', ...)` for example. The downside of using a string is that different component libraries might accidentally use the same one; using an object literal means the keys are guaranteed not to conflict in any circumstance, even when you have multiple different contexts operating across many component layers.
+We can use anything as a key — we could do `setContext('mapbox', ...)` for example. The downside of using a string is that different component libraries might accidentally use the same one; using an object literal means the keys are guaranteed not to conflict in any circumstance, even when you have multiple different contexts operating across many component layers.
+
+> Remember that context is not inherently reactive. If you need context values to be reactive then you can pass a store into context, which *will* be reactive.