Remove doc(html_root_url) attribute

[GuillaumeGomez]

I'm working on turning rust-lang/rust#82730 into an error. When I did, the build for this crate failed because when included from libstd because of this attribute. This fixes it.

[jyn514]

This doesn't seem like the right fix? it's possible to use backtrace as a dependency outside of std without enabling the std feature.

I'd suggest one of the following:

• just removing the attribute altogether: Justification of html_root_url is missing nuance api-guidelines#229
• adding a cfg or feature to backtrace-rs that's only set when building as a dependency in rust-lang/rust
• switching backtrace to being used as a crate instead of using include or path.

[GuillaumeGomez]

I think using backtrace as dependency rather than including it seems like a better approach. I'll check first why it was done this way.

[GuillaumeGomez]

This turns out to be actually quite tricky: backtrace cannot be built as a dependency of std apparently because it requires std, hence why it's included using path. I tried to go around for some time but the changes were quite massive and in the end, I'm not sure it's worth it. I decided to just remove the attribute to make things simpler for the time being. But it might be worth discussing simplifying the integration of backtrace into std.

[GuillaumeGomez]

This is ready then. Ping @JohnTitor

[GuillaumeGomez]

Thanks!