Space used in UnmanagedMemoryAccessor.WriteArray is not documented

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.