Document sizeof behavior on enum types

[heissenberg06]

Fixes #54499

Adds a bullet to the sizeof operator reference noting that sizeof applied to an enum type returns the size of its underlying integral type, that the size is evaluated at compile time, and that changing the underlying type of an enum in a referenced assembly requires recompiling consuming code to observe the new size.

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Internal previews

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docs/csharp/language-reference/operators/sizeof.md	sizeof operator - determine the memory needs for a given type

[GeeLaw]

The opening paragraph should also mention sizeof on enum types need not appear in unsafe.

[heissenberg06]

The opening paragraph should also mention sizeof on enum types need not appear in unsafe.

Good catch, thanks. Updated the opening paragraph to note that sizeof on an enumeration type is allowed in safe code (its size is a compile-time constant) and doesn't require an unsafe context. Also moved the enum link to the first mention.