Add explanation for `&mut self` method call when expecting `-> Self`

When a user tries to use a method as if it returned a new value of the
same type as its receiver, we will emit a type error. Try to detect this
and provide extra explanation that the method modifies the receiver
in-place.

This has confused people in the wild, like in
https://users.rust-lang.org/t/newbie-why-the-commented-line-stops-the-snippet-from-compiling/47322

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