CLEANUP: filters: Use the stream filter instance wording in comments

CLEANUP: filters: Use the stream filter instance wording in comments

Now that a filter configuration is called a filter instance, the per-stream
object created from it and attached to a stream is called a stream filter
instance in the documentation. Use the same wording in the comments
describing the filter instance flags, the attach/detach callbacks, the
'struct filter' and the SPOE stop callback.

The "reprensenting" typo in the 'struct filter' comment is fixed as well.