Update HIR chapter to reflect the removal of NodeIds

[LeSeulArtichaut]

See rust-lang/rust#50928.

cc @marmeladema @zackmdavis can you confirm that everything is correct? I haven't really followed the HirId-ification initiative, and I just wrote what I could find in the docs and what I could guess.

[marmeladema]

Documentation seems correct to me. One note though, NodeId are not entirely removed, they remain available / accessible before HIR lowering because that's how each AST nodes are identified. Most notably, they are heavily relied upon during name resolution passes.

EDIT: i don't know where and probably not in hir.md but i guess NodeId should still remain documented somewhere.

[mark-i-m]

@LeSeulArtichaut Would you be willing to add a new chapter about identifiers throughout the compiler and move generic info there (and maybe also stuff like #748)?

[LeSeulArtichaut]

Sure, will take a look :)

[mark-i-m]

Friendly ping @LeSeulArtichaut

[LeSeulArtichaut]

Closing in favor of #872