Document AllocateUninitializedArray security and performance traps

[jkotas]

Based on feedback from dotnet/roslyn#62864 (comment)

[opbld34]

Docs Build status updates of commit caf691e:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/GC.xml	✅Succeeded	View

For more details, please refer to the build report.

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[opbld32]

Docs Build status updates of commit 8804bc0:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/GC.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel