Cleanup: consider converting comments on private fields into xml docs

[maumar]

... so that they pop up in intellisense

[ajcvickers]

Notes from triage: we will try this, but concerns are:

• NuGet package size will increase
• It could break the docs platform

[roji]

It turns out we already have a few xmldocs on private stuff, e.g. ExecutionStrategy. These don't appear in the docs (note the protected ones which do).

It's true that these are published in the docs XML included in the nuget, but I don't really see much issue with that... This is also definitely not something we should start systematically doing on everything - just case-by-case where it makes sense.