Adding some xmldoc to the public IArgumentMatcher and IDescribeNonMatches interfaces.

Source/NSubstitute/IArgumentMatcher.cs

@@ -1,12 +1,33 @@
 namespace NSubstitute
 {
+    /// <summary>
+    /// Provides a specification for arguments for use with <see ctype="Arg.Matches (IArgumentMatcher)" />.
+    /// Can additionally implement <see ctype="IDescribeNonMatches" /> to give descriptions when arguments do not match.
+    /// </summary>
     public interface IArgumentMatcher
     {
+        /// <summary>
+        /// Checks whether the <paramref name="argument"/> satisfies the condition of the matcher.
+        /// If this throws an exception the argument will be treated as non-matching.
+        /// </summary>
+        /// <param name="argument"></param>
+        /// <returns></returns>
         bool IsSatisfiedBy(object argument);
     }
 
+    /// <summary>
+    /// Provides a specification for arguments for use with <see ctype="Arg.Matches &lt; T &gt;(IArgumentMatcher)" />.
+    /// Can additionally implement <see ctype="IDescribeNonMatches" /> to give descriptions when arguments do not match.
+    /// </summary>
+    /// <typeparam name="T">Matches arguments of type <typeparamref name="T"/> or compatible type.</typeparam>
     public interface IArgumentMatcher<T>
     {
+        /// <summary>
+        /// Checks whether the <paramref name="argument"/> satisfies the condition of the matcher.
+        /// If this throws an exception the argument will be treated as non-matching.
+        /// </summary>
+        /// <param name="argument"></param>
+        /// <returns></returns>
         bool IsSatisfiedBy(T argument);
     }
 }

Source/NSubstitute/IDescribeNonMatches.cs

@@ -2,6 +2,12 @@ namespace NSubstitute
 {
     public interface IDescribeNonMatches
     {
+        /// <summary>
+        /// Describes how the <paramref name="argument" /> does not match the condition specified by this class, or <see cref="string.Empty"/>
+        /// if a detailed description can not be provided for the argument.
+        /// </summary>
+        /// <param name="argument"></param>
+        /// <returns>Description of the non-match, or <see cref="string.Empty" /> if no description can be provided.</returns>
         string DescribeFor(object argument);
     }
 }