[WIP] improve xml doc comments for FSharp.Core

[dsyme]

There is an F# community effort going on to properly generate the FSharp.Core API docs using the fsdocs tooling, see

• https://github.com/fsharp/fsharp-core-api-docs
• https://fsharp.github.io/fsharp-core-api-docs/reference/FSharp.Core/index.html

As part of this we will need to cleanup our XML doc in the original FSharp.Core source.

This PR will contain a bunch of improvements in this regard

1. Add spacing between summary, param, exception, returns and remarks sections so it's much easier to visuall see what's what in the documentation. (currently it is hard to review and read in the source itself because it is cramped)

2. Use <see cref="..."> more systematically - we already use this in seq.fsi so there is nothing new here, but this correctly adds the T: , P: and M: tags to the references.

3. Remove a massive remarks section for the Printf module which is really written documentation, and has been transferred into the F# docs here: F# plaintext printing documentation docs#19632

See also #9805 which deals with possible improvements to the docs story where the compiler checks the docs including symbol references.

I'll push in more improvements today/tomorrow

[dsyme]

Continued in #9844