Expand on lifetime elision in impl headers

On the lifetime elision page the nomicon states:

In the future, it should be possible to elide impl headers in the same manner.

According to the edition guide that future is now:

When writing impl blocks, you can now elide lifetime annotations in some situations.

However that is about as far that explanation goes in the edition book. I found the RFC that accompanied this change which stated the following:

For impl headers, input refers to the lifetimes appears in the type receiving the impl, while output refers to the trait, if any. So impl<'a> Foo<'a> has 'a in input position, while impl<'a, 'b, 'c> SomeTrait<'b, 'c> for Foo<'a, 'c> has 'a in input position, 'b in output position, and 'c in both input and output positions.

Which seems like a reasonable explanation to include in the nomicon lifetime elision page.

Start with the Nomicon lifetime elision page linked in the issue, then compare its impl-header discussion with the linked Edition Guide and RFC explanation. Update the page to explain how input and output lifetimes are determined for impl headers, and verify that the examples and wording accurately reflect the current rules.