Lint against tabs in doc comments

[Lythenas]

Mixing spaces and tabs is always a bad idea but for doc comments it's especially bad because the rendered output might be different when using tabs or spaces.

See #4622

The actual output is/was

But the output should be

The only difference is the indentation:

With tabs:

/// 		- Recommends a signed suffix, even though the number might be too big and an unsigned
///		suffix is required
/// 		- Does not match on `_128` since that is a valid grouping for decimal and octal numbers

(depending on your editor settings the suffix ... is aligned with the - or left of it)

With spaces (and without leading whitespace):

/// - Recommends a signed suffix, even though the number might be too big and an unsigned
///   suffix is required
/// - Does not match on `_128` since that is a valid grouping for decimal and octal numbers

I'm not sure if tabs should be linted in normal rust code too but IMO it is necessary in doc comments.

[flip1995]

In normal rust code rustfmt has a config option for this IIRC. So this lint should only lint doc comments.

[FlorianRohm]

would try this as a first issue at impl Days :)