b=890528 add more documentation and make length optional when createExpectedBuffers is present r=ehsan

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extra : transplant_source : R%7DJo%87%0C%0D%0FuX%C4%BC%C0%95%08b%5D%DA%D1%BF
test for bug 890528

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extra : transplant_source : B%22%1A%B5%E9%03%D1%A4%D9%20%F7%12%FA%09%10%14%A0%19x%BA
b=890528 delay buffer must be greater than max delay frames to avoid reading what has just been written r=ehsan

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extra : transplant_source : %86L%3C%C2%F0%A79%E4w%F6u%FD%11d%A9%FCu%F90%E8
b=890528 Record silence in the delay buffer when there is no input, to avoid echos r=ehsan

--HG--
extra : transplant_source : x%A5%D0%A8%F9%7B%7DX%AA%C5%04%7E.%2C%B6%3A%EF%3F%08%5E