Clarify return values of Base64Url.Try* methods

[buyaa-n]

Summary

Improve return values of Base64Url.Try* methods, clarify it returns false if destination is too small

Relates to dotnet/runtime#108834 (comment)

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-buffers

[stephentoub]

Thanks

[learn-build-service-prod]

Learn Build status updates of commit 4dcdbb0:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Buffers.Text/Base64Url.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel