add remarks for Memory<T>.Span property

[ssg]

The cost of accessing .Span property of Memory<T> wasn't specified in the documentation.

[opbld33]

Learn Build status updates of commit 2585194:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/Memory`1.xml	✅Succeeded	View

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[BillWagner]

ping @stephentoub

[learn-build-service-prod]

Learn Build status updates of commit 98adcc7:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/Memory`1.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