Update CA1806 with Pure methods (#49544)

Author: gewarren

docs/fundamentals/code-analysis/quality-rules/ca1806.md

@@ -1,7 +1,7 @@
 ---
 title: "CA1806: Do not ignore method results (code analysis)"
 description: "Learn about code analysis rule CA1806: Do not ignore method results"
-ms.date: 06/08/2022
+ms.date: 10/30/2025
 f1_keywords:
 - CA1806
 - DoNotIgnoreMethodResults
@@ -32,6 +32,7 @@ There are several possible reasons for this warning:
 - A method that creates and returns a new string is called and the new string is never used.
 - A COM or P/Invoke method returns a `HRESULT` or error code that's never used.
 - A language-integrated query (LINQ) method returns a result that's never used.
+- A `[Pure]` method is called and the return value is never used.
 
 ## Rule description
 
@@ -41,7 +42,7 @@ Strings are immutable and methods such as <xref:System.String.ToUpper%2A?display
 
 Ignoring `HRESULT` or an error code can lead to low-resource conditions or unexpected behavior in error conditions.
 
-LINQ methods are known to not have side effects, and the result should not be ignored.
+LINQ methods and methods annotated with <xref:System.Diagnostics.Contracts.PureAttribute> are known to not have side effects, and the result should not be ignored.
 
 ## How to fix violations
 
@@ -57,7 +58,7 @@ If method A calls method B but does not use the `HRESULT` or error code that the
 
 -or-
 
-If a LINQ method A calls method B but does not use the result, use the result in a conditional statement, assign the result to a variable, or pass it as an argument to another method.
+If method A calls a LINQ or pure method B but does not use the result, use the result in a conditional statement, assign the result to a variable, or pass it as an argument to another method.
 
 ## When to suppress warnings
 
@@ -122,7 +123,7 @@ The following example fixes the violation by assigning the result of <xref:Syste
 The following example shows a method that doesn't use an object that it creates.
 
 > [!NOTE]
-> This violation cannot be reproduced in Visual Basic.
+> This violation can't be reproduced in Visual Basic.
 
 :::code language="csharp" source="snippets/csharp/all-rules/ca1806.cs" id="snippet3":::