Add cxx_flags to docs (#455)

* Add cxx_flags to docs

* Update flags to use dash in docs

Author: martinloretzzz

docs/source/builder/writing-kernels.md

@@ -257,7 +257,7 @@ are available:
   capabilities are the intersection of this list and the capabilities
   supported by the CUDA compiler. It is recommended to leave this option
   unspecified **unless** a kernel requires specific capabilities.
-- `cuda_flags` (optional): additional flags to be passed to `nvcc`.
+- `cuda-flags` (optional): additional flags to be passed to `nvcc`.
   **Warning**: this option should only be used in exceptional circumstances.
   Custom compile flags can interfere with the build process or break
   compatibility requirements.
@@ -269,9 +269,15 @@ are available:
 
 #### xpu
 
-- `sycl_flags`: a list of additional flags to be passed to the SYCL
+- `sycl-flags`: a list of additional flags to be passed to the SYCL
   compiler.
 
+### cpu
+
+- `cxx-flags`: a list of additional flags to be passed to the C++
+  compiler.
+
+
 ## Torch bindings
 
 ### Defining bindings