Clarify effect of prefetch_count

[nathanl]

Expanded to also discuss producer concurrency

[coveralls]

Pull Request Test Coverage Report for Build c6deebbe0afd38bb6f8536053a40a90aeb5fabfa-PR-92

• 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 fe8bfa570de841d1448068d70be3cab4418f1445:	0.0%
Covered Lines:	133
Relevant Lines:	175

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💛 - Coveralls

[josevalim]

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