doc: undocument uv_req_type's UV_REQ_TYPE_PRIVATE

This is merely an internal definition that may expand to OS
specific request types. It is not an actual symbolic constant
part of the enumeration.

Fixes: https://github.com/libuv/libuv/issues/1607
PR-URL: https://github.com/libuv/libuv/pull/1653
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

docs/src/request.rst

@@ -46,7 +46,6 @@ Public members
             UV_WORK,
             UV_GETADDRINFO,
             UV_GETNAMEINFO,
-            UV_REQ_TYPE_PRIVATE,
             UV_REQ_TYPE_MAX,
         } uv_req_type;