AuthHandler doc comment "default" bucket safety tips

[steveyen]

Originally from... couchbaselabs/cbgt#32

Doc improvement request, related to AuthHandler / pools.go:

// AuthHandler is a callback that gets the auth username and password
// for the given bucket.
type AuthHandler interface {
GetCredentials() (string, string, string)
}

"Should get an upgrade in the comment that specs out the contract "if using the 'default' bucket, the username should be 'default'". Either that or the more generic "the username returned by this should never be empty"."