MINOR: proxy: explicitly permit abortonclose on frontends and clarify the doc

MINOR: proxy: explicitly permit abortonclose on frontends and clarify the doc

The "abortonclose" option was recently deprecated in frontends because its
action was essentially limited to the backend part (queuing etc). But in
3.3 we started to support it for TLS on frontends, though it would only
work when placed in a defaults section. Let's officially support it in
frontends, and take this opportunity to clarify the documentation on this
topic, which was incomplete regarding frontend and TLS support. Now the
doc tries to better cover the different use cases.