doc: naming: Refine the statement about public APIs and prefixes

Clarify that the rule applies to APIs being introduced to Zephyr, and it
is not necessarily retroactive to apply to existing APIs.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

Author: carlescufi

doc/contribute/style/naming.rst

@@ -15,7 +15,7 @@ as stated in each individual sub-section.
 Public symbol prefixes
 ======================
 
-All :term:`public APIs <public API>` in Zephyr must be prefixed according
+All :term:`public APIs <public API>` introduced to Zephyr must be prefixed according
 to the area or subsystem they belong to. Examples of area or subsystem prefixes
 are provided below for reference.