Update BinaryInteger default initializer documentation
Update BinaryInteger default initializer documentation

Some time ago, it was pointed out that "truncating" would be used only for bit pattern operations. As pointed out on Swift Evolution, this is the only spot where the same term is used for dropping the fractional part of a floating point value; elsewhere, it is always spelled--even in documentation--as "rounded toward zero." This PR updates the usage here to align with existing convention.
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