[Discover] docs for background search

[florent-leborgne]

Version: 9.2 (to confirm)
Issue with context: elastic/kibana#214998
PR that enables the feature: elastic/kibana#236818

[florent-leborgne]

We agreed with the Discover team that we'll use the existing Search sessions page and update it as background search is built on the foundations of Search sessions (even though that feature was deprecated)