avoid the phrase "key property"

React components can be assigned a special property named `key`. Therefore, it's helpful to avoid the phrase "key property" unless you're talking about the `key` property :)

I'm not sold on the replacement phrase, though... I think it's solid, but two fancy words like "fundamental" and "invariant" next to each other just seem a bit unwelcoming xP then again, we're talking about some heavy technical stuff; if there aren't any lighter words that are equally precise, then these should probably stand.

docs/04-multiple-components.md

@@ -57,7 +57,7 @@ React.render(
 
 ## Ownership
 
-In the above example, instances of `Avatar` *own* instances of `ProfilePic` and `ProfileLink`. In React, **an owner is the component that sets the `props` of other components**. More formally, if a component `X` is created in component `Y`'s `render()` method, it is said that `X` is *owned by* `Y`. As discussed earlier, a component cannot mutate its `props` — they are always consistent with what its owner sets them to. This key property leads to UIs that are guaranteed to be consistent.
+In the above example, instances of `Avatar` *own* instances of `ProfilePic` and `ProfileLink`. In React, **an owner is the component that sets the `props` of other components**. More formally, if a component `X` is created in component `Y`'s `render()` method, it is said that `X` is *owned by* `Y`. As discussed earlier, a component cannot mutate its `props` — they are always consistent with what its owner sets them to. This fundamental invariant leads to UIs that are guaranteed to be consistent.
 
 It's important to draw a distinction between the owner-ownee relationship and the parent-child relationship. The owner-ownee relationship is specific to React, while the parent-child relationship is simply the one you know and love from the DOM. In the example above, `Avatar` owns the `div`, `ProfilePic` and `ProfileLink` instances, and `div` is the **parent** (but not owner) of the `ProfilePic` and `ProfileLink` instances.