Document fetch_schema netbox option

[TarantoolBot]

Product: Tarantool
Since: 2.10
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/
SME: @ olegrok

Add a more detailed description of the fetch_schema flag in the net.box doc.

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Currently user could specify "fetch_schema" flag in netbox
connection to control schema fetching from remote instance.
In case if this option is false remote spaces will be unavailable
and on_schema_reload trigger won't work.

c = netbox.connect(uri, {fetch_schema = false})
c.space -- always will be nil
c:on_schema_reload(function() end) -- never will be triggered

Requested by @olegrok in tarantool/tarantool@58625f3.