self-documenting info for documentation & source size

[robo9k]

Showing "source code size" and "documentation size" on the versioned crate pages was introduced in #2658

This could be improved by explaining how those sizes are calculated.

Taking the [email protected] example from https://toot.cat/@imperio/113415145952455653
docs.rs shows "212.28 kB"
crates.io shows "25.9 KiB"

So "source code size" is not simply size of all (source code) files in "crate size". What does it entail, what is not part of it?
For "documentation size" there is no discrepancy, but an explanation would help here aswell.

Ideally those 2 websites would also use the same byte units.

[GuillaumeGomez]

This is indeed something that should be there. Gonna fix it in the next days.