Rollup merge of #73936 - zachlute:rustdoc-optflagmulti, r=jyn514

Rustdoc: Change all 'optflag' arguments to 'optflagmulti'

Because specifying these flags multiple times will never be discernibly different in functionality from specifying them a single time, there is no reason to fail and report an error to the user.

This might be a slightly controversial change. it's tough to say, but it's hard to imagine a case where somebody was depending on this behavior, and doing this seem actively better for the user.

This originally came up in discussion of a fix for  [Cargo #8373](https://github.com/rust-lang/cargo/issues/8373), in [Cargo PR #8422](https://github.com/rust-lang/cargo/pull/8422).

The issue is that Cargo will automatically add things like `--document-private-items` to binaries, because it's the only thing that makes sense there. Then some poor user comes along and adds `--document-private-items` to their `rustdoc` flags for the project and suddenly they're getting errors for specifying a flag twice and need to track down which targets to actually add it to without getting duplicates for reasons they won't understand without deep understanding of Cargo behavior.

We're apparently hesitant to inspect `rustdoc` flags provided by the user directly in Cargo, because they're supposed to be opaque, so looking to see if it's already provided before adding it is evidently a non-starter. In trying to resolve that, one suggestion I came up with was to just change `rustdoc` to support passing the flag multiple times, because the user's intent should be clear and it's not *really* an error, so maybe this is a case of 'be permissive in what you accept'.

This PR is an attempt to do that in a straightforward manner for purposes of discussion.

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