Describe correct behavior on unexpected INTRO_ESTABLISHED extensions

IIUC, per the discussion at arti!1889 (comment 2984937) , it seems that the spec currently says that we should not reject any INTRO_ESTABLISHED message just because it has unrecognized extensions.

This is at odds with our current thinking about unrecognized extensions, though. Should we change this?

cc @mikeperry

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information