Add more remarks for `Microsoft.Extensions.Configuration` in our docs

[maryamariyan]

Comment by @KalleOlaviNiemitalo in dotnet/runtime#43869 (comment):

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While implementing Microsoft.Extensions.Configuration.IConfiguration, I did not find any documentation on the following:

• Whether the getters and setters are required to be thread-safe. I believe at least the IConfiguration.Item[String] getter is often called by multiple threads without locking.
• Whether IConfiguration.GetSection(String) is allowed to create a new configuration section and make it visible in IConfiguration.GetChildren() even if IConfigurationSection.Value is not subsequently set.
• Whether IConfiguration.GetChildren().GetEnumerator() is allowed to return an IEnumerator<IConfigurationSection> that becomes invalid if configuration values are subsequently modified or new sections are created. This relates to thread safety as callers cannot easily lock to protect themselves against such invalidation.

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Issue Details

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Comment by @KalleOlaviNiemitalo in dotnet/runtime#43869 (comment):

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While implementing Microsoft.Extensions.Configuration.IConfiguration, I did not find any documentation on the following:

• Whether the getters and setters are required to be thread-safe. I believe at least the IConfiguration.Item[String] getter is often called by multiple threads without locking.
• Whether IConfiguration.GetSection(String) is allowed to create a new configuration section and make it visible in IConfiguration.GetChildren() even if IConfigurationSection.Value is not subsequently set.
• Whether IConfiguration.GetChildren().GetEnumerator() is allowed to return an IEnumerator<IConfigurationSection> that becomes invalid if configuration values are subsequently modified or new sections are created. This relates to thread safety as callers cannot easily lock to protect themselves against such invalidation.

Author:	maryamariyan
Assignees:	-
Labels:	Pri3, area-Extensions-Configuration
Milestone:	-

[KalleOlaviNiemitalo]

This issue pertains to the IConfiguration interface. The .NET Runtime implements this interface in ConfigurationSection class by delegating to ConfigurationRoot, whose source code can be read to find out how that implementation behaves, but it is not clear whether other implementations have to behave the same way.

A custom implementation of IConfiguration could potentially avoid the performance issue dotnet/runtime#65885 by having a Dictionary<string, IConfigurationSection> children field in each node and making IConfiguration.GetChildren() return the value collection of this dictionary, but this would cause risks related to thread safety and enumerator invalidation. It would be good to document the API contract.

[maryamariyan]

@KalleOlaviNiemitalo would you be interested in improving the IConfiguration documentation by putting up a PR for this?