Update document for workaround suggestion on threadfence_system()

Change-Id: Icccab8270604a0e578a8614b9afb3f95372f4966

docs/markdown/hip_kernel_language.md

@@ -233,8 +233,7 @@ typedef struct dim3 {
 ## Memory-Fence Instructions
 HIP supports __threadfence() and  __threadfence_block().
 
-Applications that use threadfence_system can disable the L1 and L2 caches on the GPU by:
-"export HSA_DISABLE_CACHE=1".  See the hip_porting_guide.md#threadfence_system for more information.
+Support for threadfence_system() is under development.
 
 ## Synchronization Functions
 The __syncthreads() built-in function is supported in HIP. The __syncthreads_count(int), __syncthreads_and(int) and __syncthreads_or(int) functions are under development.