pkg-utils: describe the case conversion macros in more detail.

After the refactoring of the case conversion macros in dd5e620a, the
macro became even harder to understand. Improve this explanation a
little so future generations can make sense of what we're doing here.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>