Update documentation of the Mapping's size method

[wottpal]

Currently, the documentation does not say which unit the return value has (it is bytes).

CC @cmichi

[HCastano]

Can you open a PR for this?

[wottpal]

Yeah ofc. But never done rust/crate documentation before – It's just the comment/docstring above the function that needs to be changed, right?

[wottpal]

Yeah ofc. But never done rust/crate documentation before – It's just the comment/docstring above the function that needs to be changed, right?

[HCastano]

Yep, that should be all. Here's some reference links for docs in Rust:

• The Rust Book
• Rust by Example