Add docstrings to preconditions.

src/Discord.Net.Commands/Attributes/Preconditions/RequireContextAttribute.cs

@@ -11,11 +11,27 @@ namespace Discord.Commands
         Group = 0x04
     }
 
+    /// <summary>
+    /// Require that the command be invoked in a specified context.
+    /// </summary>
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
     public class RequireContextAttribute : PreconditionAttribute
     {
         public ContextType Contexts { get; }
 
+        /// <summary>
+        /// Require that the command be invoked in a specified context.
+        /// </summary>
+        /// <param name="contexts">The type of context the command can be invoked in. Multiple contexts can be speicifed by ORing the contexts together.</param>
+        /// <example>
+        /// <code language="c#">
+        ///     [Command("private_only")]
+        ///     [RequireContext(ContextType.DM | ContextType.Group)]
+        ///     public async Task PrivateOnly()
+        ///     {
+        ///     }
+        /// </code>
+        /// </example>
         public RequireContextAttribute(ContextType contexts)
         {
             Contexts = contexts;