Mention `ilog2` from `leading_zeros` docs

[yoshuawuyts]

I was chatting with @scottmcm today, who mentioned it'd be helpful to mention the newly stabilized ilog2 APIs from the leading_zeros docs. The benefit of using ilog2 is that even if the type widens, the result doesn't change.

It'd be nice if someone could contribute docs mentioning ilog2 from leading_zeros. This probably just needs to be a one-liner.

[albertlarsan68]

@rustbot claim