Add section on max_demand

[nathanl]

Small additional tweak based on https://github.com/dashbitco/broadway_rabbitmq/pull/92#discussion_r583214443

[coveralls]

Pull Request Test Coverage Report for Build a503974662ef0093e21f2be55c01840f123c94bd-PR-94

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 76.0%

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Totals
Change from base Build 49b3d70288fa8ef9b82bb5faaa7a9873612fcbf5:	0.0%
Covered Lines:	133
Relevant Lines:	175

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💛 - Coveralls

[josevalim]

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