Update JsonSerializer.xml

[PerfidusPistor]

issue finds in documentation related to functions SerializeToElement() which return a JsonElement, not a JsonDocument.

Summary

Adjusted the documentation

Fixes #Issue_Number (if available)

[dotnet-policy-service[bot]]

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

[PerfidusPistor]

@dotnet-policy-service agree

[learn-build-service-prod[bot]]

Learn Build status updates of commit 6761b2b:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System.Text.Json/JsonSerializer.xml	:white_check_mark:Succeeded	View

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