manual -> reference & formatting

'reference' sounds better than 'manual' to me here, and rust.html is
certainly wrong.

I also wrapped everything to 80 cols.
modernize code blocks
make note of language vs libraries
Remove disclaimer

This is just true of all of Rust, and doesn't make a lot of sense now.
Especially as we move towards finalizing things, I think it's time for
this to go.
'.  ' -> '. '
move keywords to table
'merican English
uhhh weird triple backticks
rust -> Rust
glob imports are an external crate
fix example
Fix terminology around boxes

it's just 'box' not 'owned box'
Remove lies about task scheduling

it's 1:1 by default now, and N:M is on its way out
runtime has no C++
no it won't
logging is an external crate
die 'managed'
vectors are not in the language
clean up some references to 'owned'
auto merge of #17402 : steveklabnik/rust/update_manual, r=brson

Because I'm still :mask: :mask: :mask: , I figured some mindless tasks would be better than trying to finish the ownership guide. 

The manual has long been waiting for some :heart: :heart: :heart: , and so I gave it a quick once-over. I made small commits in case any of the changes are a bit weird, I mostly did a few things:

1. changed 'manual' to 'reference.' I feel like this name is better. If it's not, It's not a huge deal. it shouldn't be `rust.md` though.
2. word wrapped everything appropriately. Changes 1&2 are in the first commit, so that its' easier to see the changes in the later ones.
3. fixed other small style issues
4. removed references to things that are in the standard library, and not the language itself

There's still lots of gross in here, but I didn't want to pile on too too many changes.

/cc @brson @nikomatsakis