MINOR: stream: rename "txn.redispatch" to "txn.redispatched"

MINOR: stream: rename "txn.redispatch" to "txn.redispatched"

The fetch will return true if the stream was redispatched: this is a
past action, thus we rename the fetch to better reflect its true
meaning and prevent confusions.

Documentation was updated.
While at it, the fetch was moved from internal states section to Layer 4
section, which is where it belongs.

No backport needed unless 92b2edb (" MINOR: stream: add "txn.redispatch"
fetch") gets backported.