FrozenDictionary<TKey,TValue> documentation: no mention about the thread safety of the type.

### Type of issue

Missing information

### Description

Based on my understanding, `FrozenDictionary` is a type designed to be immutable and read-only. I would have expected this type to be thread safe. I have checked [the official documentation](https://learn.microsoft.com/en-us/dotnet/api/system.collections.frozen.frozendictionary-2?view=net-8.0) of the type, but I have found no mention about its thread safety.

Based on my experience, thread safety of dotnet types is documented explicitly. For instance, the thread safety of `ImmutableDictionary` is explicitly documented [here](https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable.immutabledictionary-2?view=net-8.0#thread-safety). Given the lack of explicit documentation for `FrozenDictionary`, I must assume it as not being thread safe, even if that seems counter-intuitive given the described characteristics of that type.

Can you please clarify this point in the documentation, or point me to the correct resource if any ?

Many thanks.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.collections.frozen.frozendictionary-2?view=net-8.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Collections.Frozen/FrozenDictionary`2.xml

### Document Version Independent Id

3b4ea300-437f-c53a-0dd1-2beed0a512e4

### Article author

@dotnet-bot