doc: clarify that ``res-2`` entity does not mean 2mm
doc: clarify that ``res-2`` entity does not mean 2mm

@bbfrederick brought up the problem that, although the documentation
does not state the above explicitly, it is the most plausible reading.

This PR adds a danger admonition to clarify the ambiguity.

Follow-up: templateflow/templateflow#92.
Resolves: #1847.
Co-authored-by: Blaise Frederick <b557f486ddb9741816c81e2a2e527eff07405cf6@gmail.com>

oesteban/fmriprep,oesteban/fmriprep,oesteban/fmriprep