rustdoc: const fn fixes

[mitaa]

fixes #27362

[rust-highfive]

r? @steveklabnik

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[alexcrichton]

@bors: r+ 82d37f3

Thanks!

[bors]

⌛ Testing commit 82d37f3 with merge bce743c...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt