doc: clarify callbacks won't be called in error case

PR-URL: https://github.com/libuv/libuv/pull/918
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

docs/src/errors.rst

@@ -8,6 +8,9 @@ In libuv errors are negative numbered constants. As a rule of thumb, whenever
 there is a status parameter, or an API functions returns an integer, a negative
 number will imply an error.
 
+When a function which takes a callback returns an error, the callback will never
+be called.
+
 .. note::
     Implementation detail: on Unix error codes are the negated `errno` (or `-errno`), while on
     Windows they are defined by libuv to arbitrary negative numbers.