HIR documentation is outdated

[Arthur-Milchior]

Regarding https://rustc-dev-guide.rust-lang.org/hir.html
It references the find method, which was renamed in 24f009c5e55d18c12563dd74681ca33b8a349936 into opt_hir_node which itself was deleted in a61019b29021b7635113b48e470dbac724c81c76

As far as I understand the commits I see, the assertion

Most of the time when you are working with the HIR, you will do so via the HIR Map, accessible in the tcx via tcx.hir()

became false. Those methods to navigate the HIR are now directly in Tcx. I don't know whether it's always the case or only on those examples.

I'll plainly admit I don't know enough about rustc or the HIR to know what is relevant in the documentation, so I won't create a PR myself.