provide manpages for shell builtins

For users who are new to console usage it might be hard to figure out shell builtins. In some cases it seems not overly wrong to break the deal that manpages ought to refer to commands and either provide pages for shell builtins or - better - refer to the `SHELL BUILTIN COMMANDS` section of the manpage of `bash` (or to the section of the builtin directly - this is more confusing because the user has to scroll up to the section on builtins to figure out why this section has been opened, but provides easier access to usage information).

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: bash 4.3-11ubuntu2
Uname: Linux 4.0.0-040000-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Apr 25 20:51:16 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (22 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: bash
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)