document difference from `format_args!`

[matklad]

From the description, it sounds like this does the same thing as format_args. I think the difference is that format_args borrows the arguments, while lazy_format moves them, and avoids lifetimes. Would be helpful to exaplain that in the docs!

[Lucretiel]

Correct; this library was created specifically because format_args only ever borrows its arguments and is therefore difficult to compose (for instance, you can never usefully use it in iterator.map). This is a good idea.