Interesting case of the smart pointer inherent methods guideline

[dtolnay]

rust-lang/rfcs#1860 (comment) gives an interesting reason to consider not following the C-SMART-PTR guideline. In that case we decided to follow the guideline anyway, even though the motivation for the guideline does not apply. Let's bring up this case in the explanation of C-SMART-PTR and provide a more complete justification.