📝 Explain the W widening suffix in the API section

State that the `W` suffix means widening and what that widens: the
matcher's return value goes from one common `R` to the union of every
handler's return type.

README.md

@@ -23,7 +23,8 @@ for the design decisions and [Caveats](#caveats) for the limits.
 
 The package exports four factories and the builder types they return. Each
 factory comes in a _strict_ variant (one common return type `R`) and a
-_widening_ variant (the union of every handler's return type):
+_widening_ variant: the `W` suffix means **widening** — the return value is
+widened from one common `R` to the union of every handler's return type.
 
 | Factory                          | Builder type                    | Return of the matcher            |
 | -------------------------------- | ------------------------------- | -------------------------------- |