Applying review comments

Default constructor, copy constructor and destructor are explicitly declared as
defult.
Add clarification that the extension is an optional kernel feture
Rounding mode is specified
Minor improvement to the example
Fix a typo

Author: AlexeySotkin

sycl/doc/extensions/Bf16Conversion/SYCL_INTEL_bf16_conversion.asciidoc

@@ -65,6 +65,12 @@ The purpose of conversion from float to bfloat16 is to reduce ammount of memory
 required to store floating-point numbers. Computations are expected to be done with
 32-bit floating-point values.
 
+This extension is an optional kernel feature as described in
+https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:optional-kernel-features[section 5.7]
+of the SYCL 2020 spec. Therefore, attempting to submit a kernel using this
+feature to a device that does not support it should cause a synchronous
+`errc::kernel_not_supported` exception to be thrown from the kernel invocation`
+command (e.g. from `parallel_for`).
 
 == Feature test macro
 
@@ -97,12 +103,14 @@ enum class aspect {
 If a SYCL device has the `ext_intel_bf16_conversion` aspect, then it natively
 supports conversion of values of `float` type to `bfloat16` and back.
 
-If the device doesn't have the aspect, objects of `bfloat16` class ust not be
+If the device doesn't have the aspect, objects of `bfloat16` class must not be
 used in the device code.
 
 == New `bfloat16` class
 
-The following class provides the conversion functionality:
+The `bfloat16` class below provides the conversion functionality. Conversion
+from `float` to `bfloat16` is done with round to nearest even(RTE) rounding
+mode.
 
 [source]
 ----
@@ -117,11 +125,15 @@ bfloat16 {
   storage_t value;
 
 public:
+  bfloat16() = default;
+  bfloat16(const bfloat16&) = default;
+  ~bfloat16() = default;
+
   // Direct initialization
-  bfloat16(const storage_t& a)
+  bfloat16(const storage_t&)
 
   // Convert from float to bfloat16
-  bfloat16(const float& a);
+  bfloat16(const float&);
 
   // Convert from bfloat16 to float
   operator float() const;
@@ -140,6 +152,11 @@ public:
 
 [source]
 ----
+#include <sycl/sycl.hpp>
+#include <sycl/ext/intel/experimental/bfloat16.hpp>
+
+using sycl::ext::intel::experimental::bfloat16;
+
 bfloat16 operator+(const bfloat16 &lhs, const bfloat16 &rhs) {
   return static_cast<float>(lhs) + static_cast<float>(rhs);
 }
@@ -150,7 +167,7 @@ float foo(float a, float b) {
   bfloat16 B {b};
 
   // Convert A and B from bfloat16 to float, do addition on floating-pointer
-  // numbers, then convert the result to bfloat16 and store in C.
+  // numbers, then convert the result to bfloat16 and store it in C.
   bfloat16 C = A + B;
 
   // Return the result converted from bfloat16 to float.