Add remarks/seealso docs to DI Add*/TryAdd* methods in DependencyInjection.Abstractions

[Copilot]

AddSingleton/AddScoped/AddTransient and their keyed/Try variants lacked documentation explaining multiple-registration semantics and cross-references between Add* and TryAdd* overloads.

Description

• ServiceCollectionServiceExtensions.cs — Added <remarks> to all AddTransient, AddScoped, AddSingleton overloads clarifying that each call unconditionally appends a registration, GetService returns the last registered instance, and GetServices<T> retrieves all registrations.

• ServiceCollectionServiceExtensions.Keyed.cs — Same <remarks> added to all AddKeyedTransient, AddKeyedScoped, AddKeyedSingleton overloads, correctly referencing ServiceProviderKeyedServiceExtensions.GetKeyedService{T} and GetKeyedServices{T} (keyed services are not retrievable via the non-keyed GetService/GetServices APIs).

• ServiceCollectionDescriptorExtensions.cs — Added <seealso> to each TryAddTransient, TryAddScoped, TryAddSingleton overload pointing to the corresponding Add* overload.

• ServiceCollectionDescriptorExtensions.Keyed.cs — Same <seealso> additions for each TryAddKeyed* overload.

Testing

Documentation-only change; no behavioral modifications.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Docs for members in DependencyInjection.Abstractions lack exception information.</issue_title>
<issue_description>### Description

I heard feedback that folks felt the docs for AddSingleton lacked good information about when it would fail and why.

I took a look and I agree. It seems that whenever we have a method with a corresponding Try method, we should be sure to include the exception docs. That will help folks if they need to use the Try method always, or if they would prefer to just use the non-try method for simpler code and treat the exception case as an invalid configuration.

Reproduction Steps

Have a look at docs https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.dependencyinjection.servicecollectionserviceextensions.addsingleton?view=net-8.0

Expected behavior

Information about meaningful thrown exceptions exists.

Actual behavior

No information exists

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response</issue_description>

<agent_instructions>Improve documentation of methods like AddSingleton and TryAddSingleton.

For AddSingleton, explain in a remark that if AddSingleton is called multiple times for the same service type, all will be added to the collection. But calling GetService for that service type will only retrieve the last registered service. To get all of them, GetServices can be used.

For TryAddSingleton, I think adding a <seealso> to the appropriate overload of AddSingleton is sufficient.

Do this for all relevant methods, not just AddSingleton.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Docs for members in DependencyInjection.Abstractions lack exception information. #108024

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