Expose Xml documentation

Close #677.

There are two versions of `getXmlDocBySignature`, one uses Visual Studio services and the other uses XDocument and external files.

This PR mistakenly includes the other PR. I will rebase when the other gets merged.
I always add a space before each comment line for readability https://github.com/dungpa/PowerTools/blob/6bb5e3ac3a51194b73c12e477176a698260ad166/src/FSharpVSPowerTools.Core/SignatureGenerator.fs#L711. Should we keep it as it by removing that space?