C-CONV: Uses an inadequate std example

[TheDan64]

Under the C-CONV section, it states that to_* methods go from borrowed -> owned and is an expensive operation (usually involving a clone)

However, in the "more examples from the std library", it references Path::to_str, which has a signature of fn to_str(&self) -> Option<&str>. This seems like this method should have actually been named as_str according to these guidelines, and is therefore a inadequate example for to_* methods.

Edit: Only thing I can think of is maybe it's costly to validate UTF8 from the inner OsStr?

[dtolnay]

Thanks for catching this! The Path::to_str pattern is indeed not covered by the table. I added an explanation in ed624db. It would not be correct to call this as_str because the method has a non-negligible cost at runtime.