DOC: document the listener state transitions

This was done by reading all the code affecting a listener's state,
hopefully it will save some time in the future.

(cherry picked from commit 977afab3f824b8322853e27abbaaa752f601a504)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 8b135c000dc4e5d75f5a3764e93776af0c1cb861)
Signed-off-by: Willy Tarreau <w@1wt.eu>
2 files changed