Add overhead info

Author: Deepika

docs/reference/api/platform/MbedStats.md

@@ -9,6 +9,8 @@ You can use memory statistics functions to capture heap usage, cumulative stack
 - `MBED_HEAP_STATS_ENABLED`.
 - `MBED_STACK_STATS_ENABLED`.
 
+@Note: Overhead of 8 bytes is added with each memory allocation call `malloc` or `calloc` when heap memory statistics are enabled.
+
 ### Thread statistics
 
 You can use the thread statistics function `mbed_stats_thread_get_each` to capture the thread ID, state, priority, name and stack information for all active threads at runtime. To enable thread monitoring, you must build Mbed OS with the `MBED_THREAD_STATS_ENABLED` macro.

docs/reference/api/platform/MemTrace.md

@@ -16,6 +16,8 @@ You must enable the `memory-tracing-enabled` setting in the Mbed OS platform con
 
 You can use the `mbed_mem_trace_set_callback` API to set the callback for memory tracing. The callback is invoked every time you call standard allocation functions, such as `malloc`, `realloc`, `calloc` and `free`.
 
+@Note: Overhead of 8 bytes is added with each memory allocation call `malloc` or `calloc` when memory tracing is enabled.
+
 For a step-by-step guide about how to use optimize memory using runtime memory tracing, please see our [runtime memory tracing tutorial](/docs/development/tutorials/optimizing.html#runtime-memory-tracing).
 
 ### Memory tracing functions reference