Add edge-case examples to `{count,leading,trailing}_{ones,zeros}` methods

Some architectures (i386) do not define a "count leading zeros" instruction,
they define a "find first set bit" instruction (`bsf`) whose result is undefined
when given zero (ie none of the bits are set). Of this family of bitwise
operations, I always forget which of these things is potentially undefined for
zero, and I'm also not 100% sure that Rust provides a hard guarantee for the
results of these methods when given zero. So I figured there are others who have
these same uncertainties, and it would be good to resolve them and answer the
question via extending these doc examples/tests.

See https://en.wikipedia.org/wiki/Find_first_set#Hardware_support for more info
on i386 and `bsf` on zero.

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