doc: clarify behaviour of node-api adjust function

doc/api/n-api.md

@@ -6126,16 +6126,25 @@ NAPI_EXTERN napi_status napi_adjust_external_memory(node_api_basic_env env,
 * `[in] env`: The environment that the API is invoked under.
 * `[in] change_in_bytes`: The change in externally allocated memory that is kept
   alive by JavaScript objects.
-* `[out] result`: The adjusted value
+* `[out] result`: The adjusted value. This value should reflect the
+  total amount of external memory with the given `change_in_bytes` included.
+  The absolute value of the returned value should not  be depended on.
+  For example, implementations may use a single counter for all addons, or a
+  counter for each addon.
 
 Returns `napi_ok` if the API succeeded.
 
-This function gives V8 an indication of the amount of externally allocated
-memory that is kept alive by JavaScript objects (i.e. a JavaScript object
-that points to its own memory allocated by a native addon). Registering
-externally allocated memory will trigger global garbage collections more
+This function gives the runtime an indication of the amount of externally
+allocated memory that is kept alive by JavaScript objects
+(i.e. a JavaScript object that points to its own memory allocated by a
+native addon). Registering externally allocated memory may, but is not
+guaranteed to, trigger global garbage collections more
 often than it would otherwise.
 
+This function is expected to be called in a manner such that an
+addon does not decrease the external memory more than it has
+increased the external memory.
+
 ## Promises
 
 Node-API provides facilities for creating `Promise` objects as described in