Rollup merge of #126103 - veera-sivarajan:improve-docs-hir-impl, r=fmease

Improve Docs for `hir::Impl` and `hir::ImplItem`

Based on https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/.E2.9C.94.20Difference.20between.20.60hir.3A.3AImplItem.60.20and.20.60hir.3A.3AImpl.60.3F/near/442650915

r​? fmease

Author: workingjubilee