This makes the `in-header`, `markdown-before-content`, and `markdown-after-content` options available to `rustdoc` when generating documentation for any crate.
Before, these options were only available when creating documentation *from* markdown. Now, they are available when generating documentation from source.
This also updates the `rustdoc -h` output to reflect these changes. It does not update the `man rustdoc` page, nor does it update the documentation in [the `rustdoc` manual](http://doc.rust-lang.org/rustdoc.html).
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