Stabilize option_ref_mut_cloned

[mbrubeck]

Closes#43738.

[rust-highfive]

r? @joshtriplett

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[joshtriplett]

@bors r+

[bors]

📌 Commit 0539613 has been approved by joshtriplett

[Aaron1011]

The doc comment for that function still contains the #[feature] attribute, under the Examples heading:

 /// #![feature(option_ref_mut_cloned)]
/// let mut x = 12;

Since the feature is being stabilized, I believe that should be removed.

[mbrubeck]

The doc comment for that function still contains the #[feature] attribute

Fixed, thanks. r? @joshtriplett

[joshtriplett]

Good catch, thank you!

[joshtriplett]

@bors r+

[bors]

📌 Commit d04362f has been approved by joshtriplett

[alexcrichton]

@bors: rollup