Allow selectively suppressing warnings
When using markup like :envvar:, I often do not have a corresponding reference target (e.g. when referring to
PATH). This leaves me with the choice of either getting a spurious warning during build, or not tagging these variables correctly.
It would be great if Sphinx would allow me to selectively suppress such warnings. I imagine the least intrusive change would be to just introduce a list of "known missing" targets in conf.py. If a target is missing, sphinx would check this list and only issue a warning if the target is not in there.