AEADs don't go through ENGINE.

They'll probably stay that way too, so document it as being an ignored
parameter.

Change-Id: Iff385715f5413290a7186c38ea9ef2dd4fce9b38
Reviewed-on: https://boringssl-review.googlesource.com/5175
Reviewed-by: Adam Langley <agl@google.com>