Added clarification on norm(X,"nuc") and "fro"

I added the description so that people can ctrl+f "nuclear" or "Frobenius" to find them. Also added a sentence about the nuclear norm as it is currently the meaning of the "meaning" is not clear.
Merge pull request #121 from ernestryu/patch-2

Added clarification on norm(X,"nuc") and "fro"