[PATCH] extensions: document that `testedwith = 'internal'` is special

[Matt Mackall]

On Mon, 2015-05-18 at 16:52 -0400, Augie Fackler wrote:
> # HG changeset patch
> # User Augie Fackler <augie at google.com>
> # Date 1430253877 14400
> #      Tue Apr 28 16:44:37 2015 -0400
> # Node ID 77d2876726c574e4e881d81cf01c4eaab6e7f817
> # Parent  f8b602fd4643f86336c373f12d0e9d012a617602
> extensions: document that `testedwith = 'internal'` is special

Queued for default, thanks.

-- 
Mathematics is the supreme nostalgia of our time.