Fix documentation for default pastebin URL being used

[Daniel Hahler]

The man page states:

    -b [pastebin url] (default: http://pastebin.com)

But pastebinit determines the defaultPB variable according to the distribution being used.

This should be documented.

While at it, it would be nice to document how to set your own default (to use e.g. pastebin.com despite running on Ubuntu).

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: pastebinit 1.2-4ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-8.15-generic-pae 3.2.0
Uname: Linux 3.2.0-8-generic-pae i686
ApportVersion: 1.90-0ubuntu1
Architecture: i386
Date: Thu Jan 12 16:31:50 2012
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
PackageArchitecture: all
SourcePackage: pastebinit
UpgradeStatus: Upgraded to precise on 2011-12-16 (27 days ago)