R1:3a4e14431fce

Remove an obsolete comment about Mercurial SSH error behavior

Summary:
Depends on D18855. Ref T13036. This comment no longer seems to be accurate: anything we send over `stderr` is faithfully shown to the user with recent clients.

From [[ https://www.mercurial-scm.org/repo/hg/file/default/mercurial/help/internals/wireprotocol.txt | this document ]], the missing sauce may have been:

```
A generic error response type is also supported. It consists of a an error
message written to ``stderr`` followed by…