Improve std::hash docs
Fixes#29357.
For details on what exactly I've done, see the commit descriptions.
There are some things I'm not sure about, but would like to address before merging this so the issue can be closed; any feedback on these points would really be appriciated:
* [x] ~I didn't touch the module level docs at all. On the one hand, I think they could use a short overview over the module; on the other hand, the module really isn't that big and I don't know if I could really do anything beyond just duplicating the type's summaries...~
* [x] ~I feel like the module-level examples are quite long-winded and not to the point, but I couldn't really think of anything better. Any ideas?~
* [x] ~Should `Hasher` get an example for implementing it? There is one in the module documentation, but it only "implements" it via `unimplemented!` and I'm not sure what the value of that is.~
* [x] ~Should `Hasher`'s `write_{int}` methods get examples?~
If there's anything else you'd like to see in std::hash's docs, please let me know!
r? @rust-lang/docs
3f79bdc0a9