Clarify effect of orphan rule changes on From/Into

[janikrabe]

Updated documentation for std::convert and std::convert::From to reflect changes to orphan rule in Rust 1.41. It should no longer be necessary to implement Into directly, unless targeting an older version.

r? @steveklabnik

[rust-highfive]

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[Dylan-DPC-zz]

r? @Mark-Simulacrum

[Mark-Simulacrum]

I'm going to tentatively r? @joshtriplett here who may have a better handle on the exact orphan rule changes we made / their effects.

[joshtriplett]

The release notes mention:

You can now pass type parameters to foreign items when implementing traits. E.g. You can now write impl From for Vec {}.

The reference updates for the new orphan rules are at
https://github.com/rust-lang/reference/pull/703/files

I think this PR is accurate.

[joshtriplett]

As a nit, could you please reverse the two conditions (with the note about only when targeting older versions coming first), so that it's more immediately obvious most people should not do this?

[janikrabe]

@joshtriplett thanks, good point! I've done that; please let me know if you'd like me to change anything else.

[joshtriplett]

@bors r+ rollup

[bors]

📌 Commit d2fe7a7 has been approved by joshtriplett