Scheduler pre block limit note

[muharem]

Scheduler has the upper limit for the number of the calls scheduled per block. The benchmarks of the pallet_referenda and pallet_conviction_voting do not respect the limit and overflows it if the scheduler MaxScheduledPerBlock not high enough.

For pallet_referenda benchmarks the limit reached if track.max_deciding > scheduler.max_scheduled_per_block OR referenda.max_queued > scheduler.max_scheduled_per_block OR track.max_deciding + referenda.max_queued > scheduler.max_scheduled_per_block

For pallet_conviction_voting benchmarks the limit reached if number_of_registered_votes > scheduler.max_scheduled_per_block

This PR is an alternative solution to a more complex one (https://github.com/paritytech/substrate/pull/13143), helping a user to find a workaround faster.