Add info about IDs in the compiler

[mark-i-m]

We should add the following content about IDs:

ID memo:

• LocalDefId - the best variant, if you can use it for something do it. Cannot address entities in other crates and fine-grained entities like individual expressions in the current crate. Can address any items.
• DefId - the second choice. Can address entities in other crates, but cannot address fine-grained entities like individual expressions.
• HirId - Can address fine-grained entities like individual expressions in the current crate. Fine-grained entities from other crates can never be addressed in rustc.
• NodeId - The last choice. Can address fine-grained entities like individual expressions when HirIds are not yet created.

Originally posted by @petrochenkov in rust-lang/rust#73291 (comment)