Update PureAttribute documentation to mention CA1806 analyzer enforcement (#11949)

Author: Copilot

xml/System.Diagnostics.Contracts/PureAttribute.xml

@@ -56,7 +56,7 @@
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 ## Remarks  
- Methods and types that are marked with this attribute can be used in calls to <xref:System.Diagnostics.Contracts.Contract?displayProperty=nameWithType> methods. Pure methods do not make any visible state changes. This attribute is not enforced by the current analysis tools; you should use this attribute only if you are sure that the methods are pure.  
+ Methods and types that are marked with this attribute can be used in calls to <xref:System.Diagnostics.Contracts.Contract?displayProperty=nameWithType> methods. Pure methods do not make any visible state changes. The [CA1806](/dotnet/fundamentals/code-analysis/quality-rules/ca1806) code analyzer checks for this attribute and emits a diagnostic when the return value of a pure method isn't used.  
   
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