Clarify Validity::definitely_all_null doc semantics

Spell out that the method reports whether the validity is definitely
all-null, that a `false` result does not prove any value is valid, and
point callers at executing the validity into a `Mask` and checking
`Mask::all_false` for a definitive answer.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Author: claude

vortex-array/src/validity.rs

@@ -123,12 +123,14 @@ impl Validity {
         matches!(self, Self::NonNullable | Self::AllValid)
     }
 
-    /// Returns `true` if this validity is *definitely* all-null, i.e. it is
-    /// [`Validity::AllInvalid`].
+    /// Returns `true` if this validity is *definitely* all-null (every value is null), i.e. it
+    /// is [`Validity::AllInvalid`].
     ///
-    /// Returning `false` does not prove the presence of valid values: a [`Validity::Array`] may
-    /// still resolve to all-null once executed. Callers must treat `false` as "unknown
-    /// without compute". This is the all-null counterpart to [`Self::definitely_no_nulls`].
+    /// Returning `false` does not prove that any value is valid: a [`Validity::Array`] may still
+    /// resolve to all-null once executed. Callers must treat `false` as "unknown without
+    /// compute". For a definitive answer, execute the validity with [`Self::execute_mask`] and
+    /// check whether the resulting [`Mask`] is all-false (`Mask::all_false`). This is the
+    /// all-null counterpart to [`Self::definitely_no_nulls`].
     #[inline]
     pub fn definitely_all_null(&self) -> bool {
         matches!(self, Self::AllInvalid)