branches topic should explain bound branches

[Ian Clatworthy]

'bzr help branches' (which becomes http://doc.bazaar.canonical.com/bzr.dev/en/user-reference/branches-help.html in the User Reference) should explain bound branches. It may also be worth comparing them to heavyweight checkouts as the differences are subtle and confuse many people. See https://lists.ubuntu.com/archives/bazaar/2010q1/065891.html for an explanation of the two and how they compare.