Space used in UnmanagedMemoryAccessor.WriteArray is not documented

[agocke]

The important bit of this issue is here: https://referencesource.microsoft.com/#mscorlib/system/runtime/interopservices/safebuffer.cs,334

WriteArray is specified to take an array and write it to an unmanaged buffer. However, the space used for each element of the array is not the standard Marshal.SizeOf, it's an internal-only Marshal.SizeOfAligned, which can be significantly different. This means that callers can't know how much space is actually required to write the array without reading the source code themselves.

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