In macro expansion section, explains why import and collections must be redone

In https://rustc-dev-guide.rust-lang.org/macro-expansion.html , I'd suggest adding slightly more explanations to the part 2.1 and 2.2. I can guess why they are here but I admit I'm far from certains of being right.

2.1. I would have assume that you don't need to resolve imports at each iteration; that would be wasted work. The only exception I could think of would be if, while expanding a macro, you create new imports. But in this case, 2.4.3. may even consider this case with "integrating its output"

2.2. Similarly, I'd expect that all macro invocations were collected either at step 1, or or at 2.4.3. Unless there is a difference between "unresolved macro" and "macro invocation". In which, I'd really appreciate a clarification.

Read the macro expansion section at rustc-dev-guide.rust-lang.org/macro-expansion.html, focusing first on parts 2.1 and 2.2 and their relationship to 2.4.3. Clarify why imports and macro invocations are revisited, including whether unresolved macros differ from macro invocations, so the guide explains the behavior requested in the issue.