[MLIR][NFC] fix memref type doc comment on dynamic shape

[bondhugula]

Summary

Fix memref type doc comment to state that -1 indicates a dynamically
shaped dimension and not any negative number.

Signed-off-by: Uday Bondhugula <[e-mail]>

mlir/include/mlir/IR/StandardTypes.h

public:
   static bool classof(Type type) {
     return type.getKind() == StandardTypes::MemRef ||
            type.getKind() == StandardTypes::UnrankedMemRef;
   }
 };
 
 /// MemRef types represent a region of memory that have a shape with a fixed
 /// number of dimensions. Each shape element can be a non-negative integer or
-/// unknown (represented by any negative integer). MemRef types also have an
-/// affine map composition, represented as an array AffineMap pointers.
+/// unknown (represented by -1). MemRef types also have an affine map
+/// composition, represented as an array AffineMap pointers.
 class MemRefType : public Type::TypeBase<MemRefType, BaseMemRefType,
                                          detail::MemRefTypeStorage> {
 public:
   /// This is a builder type that keeps local references to arguments. Arguments
   /// that are passed into the builder must out-live the builder.
   class Builder {
   public:
     // Build from another MemRefType.