Clarify use of underscores in HACKING

[Aaron Bentley]

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Ian Clatworthy wrote:
> This patch is a small change to HACKING to better reflect the convention
> used around underscores. I had been using an underscore to mean
> "internal to this module" and a lack thereof to mean "external to this
> module" when Robert explained to me that underscore really means
> "internal to bzrlib".

I understand it as analogous to the "protected" keyword in C++.  These
symbols may be used by subclasses, but should not be used from outside
their class.

Yes, Tree._iter_changes is a shining exception.  There are a few others,
too.  But in general, we should not be using _members outside their class.

1. Such members are subject to change
2. If we need to use such a member outside its class, it's reasonable to
   expect that other bzrlib users will also need this.  Therefore, we
   should provide something stable for everyone.

Aaron
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