Documentation updates for AMDSMI_GPU_METRICS_CACHE_MS (#564)

Signed-off-by: Maisam Arif <Maisam.Arif@amd.com>

docs/how-to/amdsmi-cpp-lib.md

@@ -21,6 +21,12 @@ variable to the directory containing ``librocm_smi64.so`` (usually
 ``/opt/rocm/lib``) or by passing the ``-lamd_smi`` flag to the compiler.
 ```
 
+```{note}
+The environment variable ``AMDSMI_GPU_METRICS_CACHE_MS`` may be set to
+control the internal GPU metrics cache duration (ms). 
+Default 1, set to 0 to disable.
+```
+
 ```{seealso}
 Refer to the [C++ library API reference](../reference/amdsmi-cpp-api.md).
 ```