Adjectives are not instructions
"Friendly but professional" describes a mood, not a decision. It does not tell a writer whether to use contractions, how long a sentence should run, whether to lead with the benefit or the mechanism.
Every one of those is a real choice someone has to make, and a voice guide that ducks them leaves the choice to chance.
Show the before and after
The most useful page in any voice guide is a table: what someone would naturally write, next to how it should read, with a line explaining the change.
It teaches by pattern rather than by definition, and it works for humans and for prompt libraries alike.
Name what you refuse to say
A banned-phrase list is often more clarifying than the aspirational half. If "seamless," "world-class," and "solutions provider" are off the table, the writing improves before anyone has learned the theory.
Constraints travel better than ideals.