Improve docs of broker pallet (#3980)

Small adjustments which should make understanding what is going on much
easier for future readers.

Initialization is a bit messy, the very least we should do is adding
documentation to make it harder to use wrongly.

I was thinking about calling `request_core_count` right from
`start_sales`, but as explained in the docs, this is not necessarily
what you want.

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Co-authored-by: eskimor <eskimor@no-such-url.com>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Dónal Murray <donal.murray@parity.io>