Update Scheduler Pallet Documentation

[snowmead]

Description

Updates the documentation of the Scheduler Pallet based on pallet documentation guidelines.

Warning section added to inform users of some anti-patterns when scheduling runtime calls due to misconceptions of the pallet.

Closes #14715

[cla-bot-2021[bot]]

User @snowmead, please sign the CLA here.

[muharem]

there is a new documentation guidelines you might wanna check https://github.com/paritytech/substrate/blob/master/docs/DOCUMENTATION_GUIDELINES.md

[cla-bot-2021[bot]]

User @kilogold, please sign the CLA here.

[gilescope]

bot merge