Add netrc file search information to authentication documentation

Author: pszlazak

docs/user/authentication.rst

@@ -44,6 +44,9 @@ set with `headers=`.
 If credentials for the hostname are found, the request is sent with HTTP Basic
 Auth.
 
+Requests will search for the netrc file at `~/.netrc`, `~/_netrc`, or at the path
+specified by the `NETRC` environment variable.
+
 
 Digest Authentication
 ---------------------