Fix some holes in documentation

[benjamin-cates]

Hello!

Added some missing documentation, mostly examples and filling in holes in the error module.

Change list:

• Added examples to error::EmptyError, error::Cheap, error::Simple, Error::Rich
• Added examples to Parser::into_iter, Parser::to_slice
• Added note comparing IterParser::Collect and Parser::to_slice on including/excluding ignored patterns.
• Added links to relevant attribute types in errors
  • It wasn't really clear what error::Rich was storing, especially that the links to RichPattern and RichReason were buried in method headers below, so I thought it would be good to add links to those.
  • It also wasn't clear what the return type of each of the "span" methods was, since they all said they returned "&S", but for some reason, cargo doc linked to https://doc.rust-lang.org/std/primitive.reference.html 😭
• Fixed inconsistency between docs on Simple::found and Rich::found

Let me know if I should make any changes

[zesterer]

I feel like this is a somewhat contrived example since the code could be expressed equivalently but more simply like so:

let list = letter.repeated().collect_exactly::<[char; 4]>();

A better example might be something like:

// Parses whole integers
let num = text::int().padded().map(|x| x.parse::<u64>().unwrap());

// Parses a range like `0..4` into a vector like `[0, 1, 2, 3]`
let range = num.ignore_then(just("..")).then(num)
    .map(|(x, y)| x..y)
    .into_iter()
    .collect::<Vec<_>>();

// Parses a list of numbers into a vector
let list = num.separated_by(just(',')).collect::<Vec<_>>();

let set = range.or(list);

assert_eq!(set.parse("0, 1, 2, 3").unwrap(), [0, 1, 2, 3]);
assert_eq!(set.parse("0..4").unwrap(), [0, 1, 2, 3]);

[zesterer]

Thanks for the PR! I think these changes are great. I've just one suggestion regarding one of the examples.