Add more detailed comment for clearing the last nibble for odd-len NibblePath

[AryanGodara]

if (((odd + length) & 1) != 0)
    {
        ref var oldest = ref destination[spanLength];
        oldest = (byte)(oldest & 0b1111_0000);
    }

return spanLength + PreambleLength;

This portion of WriteImpl was doing an operation which was difficult to understand at first glance and needed a more detailed comment to explain.

Reference :-

If the sum of odd and length is odd, the function clears the lower nibble of the last byte in the written path.

ref var oldest = ref destination[spanLength];
gets a reference to the byte at position spanLength in the destination buffer.

oldest = (byte)(oldest & 0b1111_0000);
clears the lower nibble of this byte by performing a bitwise AND with 0b1111_0000 (which is 0xF0).

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