Add documentation on `board_name` output for the chronobox binary

[DJDuque]

The board_name is really only a "human-understandable" identifier of a chronobox (cb01, cb02, cb03, cb04). There is no guarantee at all (not even in alpha-g-detector) that these names will match whatever is in the ODB. Neither the chronobox binary, nor alpha-g-detector will attempt to keep track of ODB changes, etc.

Make it explicit in the docs for both alpha-g-detector::chronobox::BoardId and the README file for the chronobox binary.

[DJDuque]

Nonetheless, make a guarantee that changing this "human-readable" name will be considered a breaking change (this guarantee is important for high-level tools like e.g. the data-handler).