Adjust documentation

compiler/rustc_error_codes/src/error_codes/E0690.md

@@ -1,19 +1,19 @@
-A struct with the representation hint `repr(transparent)` had zero or more than
-one fields that were not guaranteed to be zero-sized.
+A struct with the representation hint `repr(transparent)` had two or more fields
+that were not guaranteed to be zero-sized.
 
 Erroneous code example:
 
 ```compile_fail,E0690
 #[repr(transparent)]
-struct LengthWithUnit<U> { // error: transparent struct needs exactly one
+struct LengthWithUnit<U> { // error: transparent struct needs at most one
     value: f32,            //        non-zero-sized field, but has 2
     unit: U,
 }
 ```
 
 Because transparent structs are represented exactly like one of their fields at
-run time, said field must be uniquely determined. If there is no field, or if
-there are multiple fields, it is not clear how the struct should be represented.
+run time, said field must be uniquely determined. If there are multiple fields,
+it is not clear how the struct should be represented.
 Note that fields of zero-sized types (e.g., `PhantomData`) can also exist
 alongside the field that contains the actual data, they do not count for this
 error. When generic types are involved (as in the above example), an error is