`Enum.IsDefined<TEnum>(TEnum)`'s documentation still mentions enum value string

`Enum.IsDefined(TEnum)`, while it only accepts an enum value, it still mentions about being able to determine whether an enum value is defined through its string name, which cannot happen in that overload.

The documentation was copied from its original, non-generic counterpart, which does implement it through an object. Well, it just wrongly refers to strings while only dealing with enum values.

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