doc: undocument uv_req_type's UV_REQ_TYPE_PRIVATE
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 <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso