Mention type placeholders in the manual and tutorial

[Kimundi]

#12764 landed, but didn't include docs for the feature.

The manual and tutorial should mention it in the context of type inference, and give a proper explanation:

• _ in a type is a "type placeholder" and has the meaning "infer this type".
• Type placeholders are not allowed in item signatures, because global inference is not allowed by design.
• Patterns like Vec<_> are called "partial type hints" and are useful in combination with things like collect() on iterators.

[steveklabnik]

The manual should have this, but I don't want to add it to the Guide.

[steveklabnik]

I want to do this, but I'm not sure where it fits in the Reference...