Editorial: Cross reference fixes

[markafoltz]

The spec refers to a context object, e.g.

The {{pictureInPictureEnabled}} attribute's getter must return `true` if
<a>Picture-in-Picture support</a> is `true` and the <a>context object</a> is
<a>allowed to use</a> the feature indicated by attribute name
`picture-in-picture`, and `false` otherwise.

I could not find a definition for context object; I think it probably means browsing context, as that is an input to the allowed to use steps.

The spec also defines the term active Picture-in-Picture session, but does not reference it anywhere (at line 188). Was it meant to be exported?

[beaufortfrancois]

@markafoltz I've opened #235 to fix all bikeshed errors.