Improve session_timeout and default_session_timeout docs

[slvrtrn]

Taken out of ClickHouse/clickhouse-js#370

It is unclear if it is seconds or milliseconds from the docs.

The source code suggests that it is in seconds: https://github.com/ClickHouse/ClickHouse/blob/fe35e6d3def4c73755e7bd20b65ebe2a2e0a52a4/src/Server/HTTPHandler.cpp#L126-L148 and https://github.com/ClickHouse/ClickHouse/blob/fe35e6d3def4c73755e7bd20b65ebe2a2e0a52a4/programs/server/config.xml#L997-L998

Perhaps sessions should also be documented in a separate section cause now it is under "default database" instead.

[Blargian]

Closed by ClickHouse/ClickHouse#74864