Update documentation for arbitrary_enum_discriminant feature

[fee1-dead]

Continuation of #639. This should only be merged after the feature is stabilized.

[fee1-dead]

@rustbot label -S-waiting-on-author

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[fee1-dead]

@rustbot label -S-waiting-on-author S-waiting-on-stabilization

[fee1-dead]

@rustbot ready

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[fee1-dead]

@rustbot label -S-waiting-on-author +S-waiting-on-review

[ehuss]

Thanks again fee1-dead, and sorry for the super long delay.