Nullable reference types in F#

[psfinaki]

Closes dotnet/fsharp#17829

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Internal previews

📄 File	🔗 Preview link
docs/fsharp/language-reference/active-patterns.md	Active Patterns
docs/fsharp/language-reference/compiler-directives.md	Compiler Directives
docs/fsharp/language-reference/compiler-options.md	Compiler Options
docs/fsharp/language-reference/fsharp-interactive-options.md	F# Interactive options
docs/fsharp/language-reference/generics/constraints.md	Constraints
docs/fsharp/language-reference/pattern-matching.md	docs/fsharp/language-reference/pattern-matching
docs/fsharp/language-reference/values/null-values.md	Null Values
docs/fsharp/style-guide/component-design-guidelines.md	F# component design guidelines
docs/fsharp/style-guide/conventions.md	F# coding conventions

[psfinaki]

@T-Gro PTAL - I didn't want to copypaste your whole blog post to one place so tried to distribute the information among the various related docs... If you think I over- or under-copied something, let me know, will update this accordingly - the null-related information shouldn't be overwhelming but should be exhaustive.

[T-Gro]

A checklist of things I expect to see in the docs (will tick the ones I identified while reviewing):

• Syntax change for type definition
• Ways of handling null (pattern match, active patterns, one or two selected library functions)
• Recommended usage (interop, vanilla F# prefers option for indicating absence of value) == guidelines
• Generic type constraints (null and not null)
• Nullable Project property
• CLI fsc.exe and fsi settings
• Conditional DEFINE added by project property

[psfinaki]

@T-Gro thanks for the checklist, all valid points, I think I have addressed the missing parts.

[psfinaki]

@BillWagner @KathleenDollard PTAL at the English side of things :)

[BillWagner]

This LGTM. I had one formatting suggestion to highlight that nullable reference types are designed for interop