Change "setup" to "set up" (semantics of how it's used here).

Lose the "so" at the end of this line.  Reads better.

I assume the mismatched indent here is for proper formatting behavior?

Might read better if rephrased something like "Do not put anything after the <code>=</code> so that Mercurial will look in the default places for the extensions."

Change "as well as" to "and".

Lose the comma here.

Should be "Review Board" for consistency with the rest of the text.

The comma here isn't strictly necessary.  Not opening an issue here because I can't decide if it reads better with or without the comma.

I'd suggest something more like:
Once you have an account and have logged into our Review Board site, you can begin using RBTools.  In your shell, navigate to a Mercurial clone of one of the Pidgin or purple-related projects, then run the <code>rbt login</code> command.  You should only need to do this once, unless you change your password or have run the <code>rbt logout</code> command.

Lose the comma

Do you actually mean to say dry-run here?  The rest of this paragraph talks about the actual rebase.

Lose "back"

Lose the second "just"

Looks good.  After the last change I note below, should be good to bo.

I missed this in the initial review, but the comma after "branching point" shouldn't be here either.

Ship It!