Document a behavior of string.Create

[tannergooding]

This resolves https://github.com/dotnet/runtime/issues/106903 by documenting the agreed upon behavior:

• We're good at guaranteeing that the span points to memory that has space for a least one more element than the span
• We'd guarantee that the span points to zero'd memory, including the null terminator.
• Our docs would say that if you write to the extra location it must be a null terminator. Writing anything else will result corrupting the string and thus is undefined behavior.

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Learn Build status updates of commit fcc3a28:

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

Learn Build status updates of commit c811318:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System/String.xml	:white_check_mark: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