wireproto: document the wonky push protocol for SSH
It took me several minutes to figure out how the "unbundle"
protocol worked. It turns out that the SSH protocol handler
sends an empty reply that is interpreted as "OK to send" and
only then does the client send the bundle payload.
On top of that, the response is different depending on whether
the operation was successful or not. I nearly pulled out my hair
deciphering this.
Differential Revision: https://phab.mercurial-scm.org/D2385
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