Clarify masking of bits in Field::random impls

Co-Authored-By: Daira Hopwood <daira@jacaranda.org>

ff/ff_derive/src/lib.rs

@@ -892,7 +892,7 @@ fn prime_field_impl(
                         #name(#repr(repr))
                     };
 
-                    // Mask away the unused bits at the beginning.
+                    // Mask away the unused most-significant bits.
                     tmp.0.as_mut()[#top_limb_index] &= 0xffffffffffffffff >> REPR_SHAVE_BITS;
 
                     if tmp.is_valid() {

sapling-crypto/src/jubjub/fs.rs

@@ -292,7 +292,7 @@ impl Field for Fs {
                 Fs(FsRepr(repr))
             };
 
-            // Mask away the unused bits at the beginning.
+            // Mask away the unused most-significant bits.
             tmp.0.as_mut()[3] &= 0xffffffffffffffff >> REPR_SHAVE_BITS;
 
             if tmp.is_valid() {