Add note that SafeCanonicalizationMethods is static-like.

[vcsjones]

Summary

This property, for historical purposes, is instance-based but is backed by a static field, so changes are observed between instances.

Fixes dotnet/runtime#46256

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Issue Details

---

Summary

This property, for historical purposes, is instance-based but is backed by a static field, so changes are observed between instances.

Fixes dotnet/runtime#46256

Author:	vcsjones
Assignees:	-
Labels:	area-System.Security
Milestone:	-

[bartonjs]

Technical content LGTM. @gewarren do you have any style or language changes you'd like to see?

[opbld30]

Docs Build status updates of commit 9400daa:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
xml/System.Security.Cryptography.Xml/SignedXml.xml	⚠️Warning	View	Details

xml/System.Security.Cryptography.Xml/SignedXml.xml

• Line 0, Column 0: [Warning-xref-not-found] Cross reference not found: 'System.Security.Cryptography.Xml.SignedXml*'.

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[opbld33]

Docs Build status updates of commit 4573f55:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Security.Cryptography.Xml/SignedXml.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.

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[gewarren]

LGTM. I'll merge it now; thanks!