doc: Use vec![] instead of vec!() in Vec.
doc: Fill vec documentation with examples.

Add more useful functions to main example.
doc: Vec::from_raw_parts.

Also more explicit raw pointer handling in unsafe examples.
doc: Normalize example style

Remove unnecessary `use std::vec::Vec`. Use ``` code blocks
with rust as default instead of spelling it out.
auto merge of #15894 : treeman/rust/vec-doc, r=alexcrichton

Fill in examples for missing methods. Opt for `vec![]` instead of `vec!()`.