Minor `transcoding()` doc improvement.

Moved the note about the contract to the Params: section, which until now was pretty useless.

A user checking the function's parameters  will usually look at `Params:` first and may miss any text about e.g. requirements of a parameter if it's not mentioned there in `Params:`.
Merge pull request #2819 from kiith-sa/patch-3

*trivial* `DDoc` Minor `transcode()` doc improvement.