* Update portable pdb guidance in 4.7.1
Unfortunately due to https://github.com/Microsoft/dotnet/issues/529 we had to withdraw Portable PDB from 4.7.1 but we anticipate having the feature restored in 4.7.2 with much improved performance.
* Update portable_pdb.md
Clarify that in 4.7.2 the issue is targeting, not just what runtime the code is executing on