samples: synchronization: fix comments & structure

This commit has no functional changes. It makes the comments up to date
with the code and changes the code structure to ensure consistency.

These inconsistencies were introduced when the static thread definition
was replaced with dynamic, but the comments remained unchanged.

 - Fixed outdated comments about the static approach that were
   not corresponding to the code.
 - Rearrange thread definitions, to obtain similar code structure
   for both threads.

Signed-off-by: Vilem Gottwald <xvigo.dev@gmail.com>