Require precipitation timing in generated text

docs/internal/generatedtext.md

@@ -17,8 +17,9 @@ value and canonical normalized JSON, loads its canonical schema through
 
 Daily, Today, and Tomorrow use a day-style value with required trimmed summary
 and one or more nonblank discussion paragraphs. Hourly requires trimmed summary
-and a single trimmed discussion string. Each form permits optional trimmed
-precipitation-timing and confidence prose. Typed decoding rejects unknown JSON
+and a single trimmed discussion string. Every form also requires the
+`precipitation_timing` field; an empty string means there is no supported timing
+prose to render. Typed decoding rejects missing required fields and unknown JSON
 fields; no general-purpose JSON Schema engine is used at runtime.
 
 ## Render contexts