Add note about ValueTask and ValueTask<TResult> default constructors (#4397)

[glihm]

Fixes #4397

Summary

As described in the issue, ValueTask and ValueTask<TResult> are lacking of information about default constructors behavior.
This PR is adding a note reworded from the code itself.

[tarekgh]

I am not sure it is a good idea to document the default constructor that is automatically generated by the compiler. Usually users shouldn't use the default constructor. and If using the default value, should be used to indicate if the instance is not initialized or not similar to null for the instance types. This is a generic issue for the whole framework structs.

I'll let @stephentoub comment on that but his response may be delayed a little.

[stephentoub]

Documenting what the status of a zero-initialized ValueTask/ValueTask<T> is fine, though it's not really using a "default constructor" when it does so, so I'd be careful with such language.

[glihm]

@tarekgh @stephentoub thanks for the feedback, I'll try a rewording to avoid such confusion. 👍🏻

[glihm]

In struct documentation, those lines are written:

You can't declare a parameterless constructor.
Every structure type already provides an implicit parameterless constructor that produces the default value of the type.

changing default to parameterless constructor can be less confusing and in the same spirit of struct semantic, what do you think?

[tarekgh]

changing default to parameterless constructor can be less confusing and in the same spirit of struct semantic, what do you think?

I think this can be reasonable but I am not expecting users doing ValueTask v = default will always relate to the parameterless constructor. maybe we can have something like:

An instance created with parameterless constructor or by the default(ValueTask) syntax (a zero initialized structure) represents a synchronously, successfully completed operation. I am sure @stephentoub can rephrase it better :-)

[carlossanlop]

@glihm any chance you can follow up?

[adamsitnik]

LGTM, thank you @glihm !

[opbld30]

Docs Build status updates of commit a4e6c60:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Threading.Tasks/ValueTask.xml	✅Succeeded	View
xml/System.Threading.Tasks/ValueTask`1.xml	✅Succeeded	View

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