fix simd_bitmask docs

crates/core_simd/src/intrinsics.rs

@@ -132,9 +132,10 @@ extern "platform-intrinsic" {
     // `fn simd_bitmask(vector) -> unsigned integer` takes a vector of integers and
     // returns either an unsigned integer or array of `u8`.
     // Every element in the vector becomes a single bit in the returned bitmask.
-    // If the vector has less than 8 lanes, a u8 is returned with zeroed trailing bits.
     // The bit order of the result depends on the byte endianness. LSB-first for little
     // endian and MSB-first for big endian.
+    // If the vector has less than 8 lanes, the mask lives in the least-significant bits
+    // (e.g., [true, false] becomes `0b01` on little endian and `0b10` on big endian).
     //
     // UB if called on a vector with values other than 0 and -1.
     #[allow(unused)]