doc: doc lifetime of n-api last error info

Document the lifetime of the structure returned
by napi_get_last_error_info

Fixes: nodejs/abi-stable-node#251

Author: mhdawson

doc/api/n-api.md

@@ -269,6 +269,9 @@ Returns `napi_ok` if the API succeeded.
 This API retrieves a `napi_extended_error_info` structure with information
 about the last error that occurred.
 
+*Note*: The content of the 'napi_extended_error_info` returned is only
+valid up until an n-api function is called on the same `env`.
+
 *Note*: Do not rely on the content or format of any of the extended
 information as it is not subject to SemVer and may change at any time.
 It is intended only for logging purposes.