Mention that Ember prototype extensions are optional early.

While reading the Ember guides, the code samples in `Computed Properties` made me uneasy, as it seemed to me that Ember modifies Function's prototype without warning.  When I got to `Observers`, I saw that prototype extensions are optional. That made me happy. I's very nice for adding Ember to a large existing codebase.

This change:
1) Adds the language around prototype modification found in `Observers` to `Computed Properties`, to put other reader's minds at ease as soon as the issue surfaces.
2) Links to the relevant Ember configuration section, so interested readers can check it out right away.