[SYCL][Doc] Add isnan to bfloat16 math doc

sycl/doc/extensions/experimental/sycl_ext_oneapi_bfloat16_math_functions.asciidoc

@@ -55,13 +55,13 @@ specification.
 
 == Overview
 
-This extension adds `bfloat16` support to the `fma`, `fmin`, `fmax` and
-`fabs` SYCL floating point math functions. These functions can be used as
+This extension adds `bfloat16` support to the `fma`, `fmin`, `fmax`, `fabs`
+and `isnan` SYCL floating point math functions. These functions can be used as
 element wise operations on matrices, supplementing the `bfloat16` support
 in the sycl_ext_oneapi_matrix extension.
 
-The descriptions of the `fma`, `fmin`, `fmax` and `fabs` SYCL floating point
-math functions can be found in the SYCL specification:
+The descriptions of the `fma`, `fmin`, `fmax`, `fabs` and `isnan` SYCL floating
+point math functions can be found in the SYCL specification:
 https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#_math_functions.
 
 == Specification
@@ -104,6 +104,22 @@ The following functions are only available when `T` is `bfloat16` or
 `sycl::marray<bfloat16, {N}>`, where `{N}` means any positive value of
 `size_t` type.
 
+==== isnan
+
+```c++
+namespace sycl::ext::oneapi::experimental {
+
+bool isnan(bfloat16 x);
+
+template <size_t N>
+sycl::marray<bool, N> isnan(sycl::marray<bfloat16, N> x);
+} // namespace sycl::ext::oneapi::experimental
+```
+
+===== Description
+
+Returns true if x is NAN value, otherwise returns false.
+
 ==== fma
 
 ```c++