API Docs: std::floating point stuff

[steveklabnik]

Part of #29329

like http://doc.rust-lang.org/std/f32/ pretty sure these are generated too, for all of them.

Here's what's needed to close out this issue:

• the module docs should have a bit more about why this module exists.

[steveklabnik]

I am happy to mentor anyone who wants to tackle this issue.

[palango]

Hej @steveklabnik , what do you think about the following description?

This module provides access to properties and bounds of this floating point data type. Furthermore mathematical constants are provided in the consts module.

[steveklabnik]

Hey @palango,

I like it. We should ask @rust-lang/libs though, why are some of these constants in consts and some are not?

[palango]

I guess the std::f32 module is for data-type specific constants, whereas std::f32::consts is for mathematical constants with the possible accurcy.