doc: update measure memory rejection information

If in case context is unable to allocate a promise then
`ERR_CONTEXT_NOT_INITIALIZED` error will be thrown (as promise
rejection) in the vm measureMemory call.

Author: yashLadha

doc/api/vm.md

@@ -1060,8 +1060,9 @@ current V8 isolate, or the main context.
     exits before the next GC). With eager execution, the GC will be started
     right away to measure the memory.
     **Default:** `'default'`
-* Returns: {Promise} If the memory is successfully measured the promise will
-  resolve with an object containing information about the memory usage.
+* Returns: {Promise} If the memory is successfully measured, the promise will
+  resolve with an object containing information about the memory usage, else it
+  will be rejected with an error with `ERR_CONTEXT_NOT_INITIALIZED` code property.
 
 The format of the object that the returned Promise may resolve with is
 specific to the V8 engine and may change from one version of V8 to the next.