Check XML doc parameter names for FSharp.Core via opt-in checking

[dsyme]

1. Fixes mistakes in FSharp.Core XML docs

2. Fix mistakes in FCS docs

3. Implements the opt-in XML document checking /warnon:3390 and applies it to FSharp.Core,

   When /warnon:3390 is enabled (e.g. for FCS and FSharp.Core):

   • checks <param name="xyz"> and <paramref name="xyz"> it checks that xyz is a valid parameter name for the thing being documents

   • checks that if any parameter is given docs, then all are given docs

4. Adds the ability to get the elaborated (e.g. with summary tags) XML from the FCS API, which we should always have allowed. This will become more important if/when we support cross-reference cref elaboration in XML docs.

5. Fixes a bug where you couldn't add documentation to an implicit constructor, see this stack overflow question.

Aside: This PR also contains the beginning of code for cross-reference checking and elaboration, allowing the use of "short name" forms for cross references, including rename-refactor on these. However that code is not enabled.

[dsyme]

This is ready (once green - having some niggling issues)

[dsyme]

This is now ready for review

[dsyme]

@cartermp I think it makes sense to document

• /warnon:1182 - unused variables
• /warnon:3390 - check XML docs for validity and completeness

in the Microsoft F# docs. I was going to put something under "docs" but it's likely better in the actual guide

[dsyme]

Side note: I wanted to add separate warnings for

• give warning if elements do not have documentation

• give warning if an element doesn't have full parameter information

However there are some issues with that. Notably

1. It forces people to provide documentation for implicit constructors (or else have a signature file). This is awkward, e.g.

   /// The type docs
   type C
       /// The implicit constructor docs
       (x: int, y: int) = ...
   

   I don't really want a feature that will force this on people. We should probably allow a special section in the docs for the type e.g.

   /// <summary>The type docs</summary>
   /// <ctor><summary>The implicit constructor docs</summary></ctor>
   type C(x: int, y: int) = ...
   

2. It is also common to have incomplete docs (e,g, no parameter docs) in an implementation but have complete docs in a signature. So enforcement of completeness should only be done on the signature docs if a signature is present.

But those are for later. What we have in this PR is enough to pin down quality for FCS and FSharp.Core docs.

[cartermp]

How did this enormous diff occur?

[dsyme]

Not sure

[dsyme]

@KevinRansom Thank you, it's great to have this in

[dsyme]

Relevant documentation PR: dotnet/docs#20645