Document a behavior of string.Create

[tannergooding]

This resolves dotnet/runtime#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.

[dotnet-policy-service]

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

[learn-build-service-prod]

Learn Build status updates of commit fcc3a28:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/String.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

[learn-build-service-prod]

Learn Build status updates of commit c811318:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/String.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