Add units to EPSSIL and EPSSOX (and more)

[beutlich]

          > As noted in #4377 we might also document that epssil means epsilon (di-electric constant) for Silicon, and add a proper unit.

I updated the descriptions accordingly but will change the units of EPSSIL and EPSSOX in a separate PR once this PR gets merged, because this PR does not introduce any simulation relevant changes.

Originally posted by @beutlich in https://github.com/modelica/ModelicaStandardLibrary/issues/4378#issuecomment-2734537993