Clarify eager macro explanation

[Arthur-Milchior]

This CL tries to improve the text around the order in which macro are expanded in various related way.

1. Remove duplication of "smoother".

Its first occurrence is

a smoother user experience. As an example

It seems that we'll have an example of a smooth user experience. It is not the case, we have an example of macro expansion. Removing this occurrence of "smooth" help understand this won't be considered in the following example.

As the same "smooth" explanation is given below, nothing is lost.

2. Use an actual example of where eager expansion is needed

This example comes, modified, from https://doc.rust-lang.org/std/macro.compile_error.html . It uses cfg to cause a compile error if something is not as expected.

As far as my understanding of rust goes, this could not be done with call-by-name evaluation.

At the same time, this example shows the steps of rewritting. (Approximatively. I still use code represented as strings instead of a list of token/an AST, for the sake of the readability)

3. Not all macros are in the queue

According to the way step 1 of the algorithm is currently written, it seems that, in foo!(bar!(baz));, both foo!(bar!(baz)) and bar!(baz) should be in the queue. This is false, as bar! should not be evaluated until foo! is.

So the rewritting state that the traversal does not enter unevaluated macros. It also add a note about the exception of non-eager macros.

Technically, it should state that, for non-eager macro, the macro itself should not be evaluated until all of its argument containing macros are evaluated. But I guess it's already fit in the case "macro expansion is not resolved".