add-apt-repository manual does not tell all repository string forms

[Jarno Suni]

According to the manual, command
    apt-add-repository http://extras.ubuntu.com/ubuntu
is invalid, but --help tells it is valid (in examples).

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: software-properties-common 0.92.37.7
ProcVersionSignature: Ubuntu 4.4.0-51.72~14.04.1-generic 4.4.30
Uname: Linux 4.4.0-51-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.21
Architecture: amd64
CurrentDesktop: XFCE
Date: Wed Nov 30 10:15:12 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2014-09-21 (801 days ago)
InstallationMedia: Ubuntu-Studio 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.1)
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)