Document `ref` return as required for field accessors in UnsafeAccessorAttribute

[xparadoxical]

Summary

The documentation of UnsafeAccessorAttribute incorrectly suggests the ref return is optional for field accessors. Without ref you get System.BadImageFormatException: 'Invalid usage of UnsafeAccessorAttribute.'

Example accessor throwing the above exception when invoked:

[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_charBuffer")]
static extern char[] GetCharBuffer(StreamReader sr);

[learn-build-service-prod[bot]]

Learn Build status updates of commit 3f91840:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System.Runtime.CompilerServices/UnsafeAccessorAttribute.xml	:white_check_mark:Succeeded	View

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