Improve doc of some methods that take ranges

[tkr-sh]

Some methods that were taking some range in parameter were a bit inconsistent / unclear in the panic documentation.

Here is the recap:

• Replaced "start/end point" by "start/end bound" to be coherent with RangeBounds naming (it's also easier to understand I think)
• Previously, it was written "[...] or if the end point is greater than the length of [...]", but this is not entirely true! Actually, you can have a start bound that is greater than the length, with an end bound that is unbounded and it will also panic. Therefore I think that "[...] one of the range bound is bounded and greater than the length of [...]" is better!
• String methods weren't mentionning that the method panics if start_bound > end_bound but it actually does! It uses slice::range which panics when start > end. (https://doc.rust-lang.org/stable/src/alloc/string.rs.html#1932-1934, https://doc.rust-lang.org/stable/src/core/slice/index.rs.html#835-837).
  You can also test it with:

struct MyRange;
impl std::ops::RangeBounds<usize> for MyRange {
    fn start_bound(&self) -> std::ops::Bound<&usize> {
        std::ops::Bound::Included(&3usize)
    }
    fn end_bound(&self) -> std::ops::Bound<&usize> {
        std::ops::Bound::Included(&1usize)
    }
}

fn main() {
    let mut s = String::from("I love Rust!");
    s.drain(MyRange); // panics!
}

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[tkr-sh]

@rustbot label +A-docs