Harmonize HttpClientJsonExtensions exception docs with HttpClient docs

[antonfirsov]

There needs to be a PR similar to #11103 documenting all exceptions HttpClientJsonExtensions, including stored exceptions.

(Eventually we need to harmonize the way we document stored exceptions in System.Net, but I consider that to be tracked by #7840.)

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json