rustdoc failed to pick up method summary comments in some cases

[nodakai]

https://doc.rust-lang.org/nightly/std/?search=max_value

i8::max_value(), u8::max_value() and a few others have summaries as given in the source code, but i32::max_value(), u32::max_value() and a few others somehow don't.

This is perplexing when we recall the max_value() method impls are generated by macros https://github.com/rust-lang/rust/blob/master/src/libcore/num/mod.rs#L100

[GuillaumeGomez]

cc @rust-lang/dev-tools

I'll try to take a look in the next days.