IDL compiler: Document interfaces

This is the last inherited interface!
Straightforward, but affects several places in the code.

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/132223006

git-svn-id: svn://svn.chromium.org/blink/trunk@164868 bbb929c8-8fbe-4397-9dbb-9b2b20218538