doc: binary encoding is not deprecated

When v8 implemented proper one-byte string support Node's internal
"binary" encoding implementation was removed in favor of it. The result
was that "binary" encoding effectively became "latin-1" encoding.
Because of this and because one-byte strings are natively supported by
v8 the buffer encoding is not deprecated and will not be removed.

Ref: 83261e78 "deps: update v8 to 3.17.13"
PR-URL: https://github.com/nodejs/node/pull/3441


Reviewed-By: Ben Noordhuis <ben@strongloop.com>