Improve Buffer.len() return type docs

[amunra]

It's been reported that the len method can be confused to return the row count instead of the byte count.

We should improve the docs to explain that the buffer is not a collection of rows, but a binary buffer.

Additionally, the len method in Rust should use a ByteCount typedef of usize to further clarify its use.

[amunra]

N.B.

We should follow through with similar doc / tidy up for the Python __len__ dunder method and C++ .size() method.