Merge pull request dotnet#8547 from dakersnar/fix-69694

Fix documentation for INumberBase IsComplex and IsImaginary

Author: dakersnar

xml/System.Numerics/INumberBase`1.xml

@@ -259,7 +259,7 @@
 
 ## Remarks
 
-This function returns `false` for a complex number `a + bi` where `b` is zero.
+This function returns `false` for a complex number `a + bi` where `a` or `b` is zero. In other words, it excludes real numbers and pure imaginary numbers.
 
           ]]></format>
         </remarks>
@@ -355,15 +355,15 @@ A return value of `false` does not imply that <xref:System.Numerics.INumberBase%
       </Parameters>
       <Docs>
         <param name="value">The value to be checked.</param>
-        <summary>Determines if a value represents an imaginary number.</summary>
+        <summary>Determines if a value represents a pure imaginary number.</summary>
         <returns>
-          <see langword="true" /> if <paramref name="value" /> is an imaginary number; otherwise, <see langword="false" />.</returns>
+          <see langword="true" /> if <paramref name="value" /> is a pure imaginary number; otherwise, <see langword="false" />.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[
 
 ## Remarks
 
-This function returns `false` for a complex number `a + bi` where `a` is non-zero.
+This function returns `false` for a complex number `a + bi` where `a` is non-zero, as that number is not purely imaginary.
 
           ]]></format>
         </remarks>