[rustdoc] Suggest trailing `\` instead of two spaces for hard line break

[Marcono1234]

What it does

When a rustdoc comment contains two trailing spaces for a Markdown hard line break, the lint should suggest using a trailing backslash (\) instead.

rustdoc uses CommonMark, and it allows using \ for hard line breaks, see the specification.

Advantage

A trailing \ makes the intention for a hard line break clearer than two trailing spaces, which can be difficult to notice in Git diffs, and might be removed accidentally during refactoring.

Drawbacks

• The choice whether to use two spaces or a \ is subjective
  (this could be solved by adding the lint to clippy::pedantic)
• If there is not actually a need for a hard line break (e.g. if a > ... follows), then the \ might be shown literally (CommonMark example)

Example

/// First line··
/// Second line
fn foo() {}

(· indicating a space)

Could be written as:

/// First line\
/// Second line
fn foo() {}

[Jacherr]

This would probably end up being a restriction lint rather than being applied by default, but nevertheless sounds quite useful - I haven't seen the double space method used before, in all honesty.

@rustbot claim