Change AsyncReadExt::read docs formatting to clarify requirements

[AeonSolstice]

Motivation

I was reading the docs for the method, and in its current state, it read to me as:

No guarantees are provided about the contents of buf when this function is called, implementations cannot rely on [any] property [of] the contents of buf being true.

With the inline block formatting on true, it makes it seem that you must not rely on buf containing the value true.

Solution

Remove the inline block formatting on true, as we are not talking about the boolean value true in code.

[Darksonn]

Thanks.