doc: specify that errno is a number, not a string

The documentation erroneously described the errno property as an alias
for the code property, but that is not the case in the implementation.
errno is the error code of the error as a number, and code is the error
code of the error as a string.

PR-URL: https://github.com/nodejs/node/pull/9007
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

doc/api/errors.md

@@ -449,13 +449,15 @@ added properties.
 ### Class: System Error
 
 #### error.code
-#### error.errno
 
 Returns a string representing the error code, which is always `E` followed by
 a sequence of capital letters, and may be referenced in `man 2 intro`.
 
-The properties `error.code` and `error.errno` are aliases of one another and
-return the same value.
+#### error.errno
+
+Returns a number corresponding to the **negated** error code, which may be
+referenced in `man 2 intro`. For example, an `ENOENT` error has an `errno` of
+`-2` because the error code for `ENOENT` is `2`.
 
 #### error.syscall