update TargetFeature::Forbidden docs

[RalfJung]

That comment was a bit outdated, I think I wrote it before abi_required_features existed.

r? @workingjubilee
Cc @wesleywiser

[rustbot]

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[workingjubilee]

maybe we should enforce that those kinds of links are in the format that can be moved into a doc comment even if... nahhh that's too silly.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...............................F.................. (50/143)
.................................................. (100/143)
...........................................        (143/143)

======== tests/rustdoc-gui/go-to-collapsed-elem.goml ========

[ERROR] line 40
    at `tests/rustdoc-gui/go-to-collapsed-elem.goml` line 21: Error: Node is detached from document: for command `click: "//*[@id='search']//a[@href='../test_docs/struct.Foo.html#method.must_use']"`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/struct.Foo.html?search=t_use>


<= doc-ui tests done: 142 succeeded, 1 failed, 0 filtered out

Error: ()

[workingjubilee]

...I really don't think that's your fault.

[workingjubilee]

gonna try and see if we can get a clean run

[RalfJung]

...I really don't think that's your fault.

Yeah that's #93784