Replace default_stream_value with get_default_stream in docs. (#11985)

Brings the docs in line with the new way of getting the default stream in libcudf.

Authors:
   - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
   - Yunsong Wang (https://github.com/PointKernel)
   - Tobias Ribizel (https://github.com/upsj)
   - Nghia Truong (https://github.com/ttnghia)

cpp/doxygen/developer_guide/DEVELOPER_GUIDE.md

@@ -416,7 +416,7 @@ internal API in the `detail` namespace. The internal `detail` API has the same p
 public API, plus a `rmm::cuda_stream_view` parameter at the end with no default value. If the
 detail API also accepts a memory resource parameter, the stream parameter should be ideally placed
 just *before* the memory resource. The public API will call the detail API and provide
-`cudf::default_stream_value`. The implementation should be wholly contained in the `detail` API
+`cudf::get_default_stream()`. The implementation should be wholly contained in the `detail` API
 definition and use only asynchronous versions of CUDA APIs with the stream parameter.
 
 In order to make the `detail` API callable from other libcudf functions, it should be exposed in a
@@ -447,7 +447,7 @@ namespace detail{
 
 void external_function(...){
     CUDF_FUNC_RANGE(); // Generates an NVTX range for the lifetime of this function.
-    detail::external_function(..., cudf::default_stream_value);
+    detail::external_function(..., cudf::get_default_stream());
 }
 ```
 

cpp/doxygen/developer_guide/DOCUMENTATION.md

@@ -224,7 +224,7 @@ Also, \@copydoc is useful when documenting a `detail` function that differs only
      */
     std::vector<size_type> segmented_count_set_bits(bitmask_type const* bitmask,
                                                     std::vector<size_type> const& indices,
-                                                    rmm::cuda_stream_view stream = cudf::default_stream_value);
+                                                    rmm::cuda_stream_view stream = cudf::get_default_stream());
 
 Note, you must specify the whole signature of the function, including optional parameters, so that doxygen will be able to locate it.