Add explanatory comment for perfect hashing.

Author: Cesar Blum Silveira

src/Microsoft.AspNet.Server.Kestrel/Infrastructure/MemoryPoolIterator2Extensions.cs

@@ -262,6 +262,21 @@ public static ArraySegment<byte> GetArraySegment(this MemoryPoolIterator2 start,
             return new ArraySegment<byte>(array, 0, length);
         }
 
+        /// <summary>
+        /// Checks that up to 8 bytes between <paramref name="begin"/> and <paramref name="end"/> correspond to a known HTTP string.
+        /// </summary>
+        /// <remarks>
+        /// A "known HTTP string" can be an HTTP method name defined in the HTTP/1.1 RFC or an HTTP version (HTTP/1.0 or HTTP/1.1).
+        /// Since all of those fit in at most 8 bytes, they can be optimally looked up by reading those bytes as a long. Once
+        /// in that format, uninteresting bits are cleared and the remaining long modulo 37 is looked up in a table.
+        /// The number 37 was chosen because that number allows for a perfect hash of the set of
+        /// "known strings" (CONNECT, DELETE, GET, HEAD, PATCH, POST, PUT, OPTIONS, TRACE, HTTP/1.0 and HTTP/1.1, where strings
+        /// with less than 8 characters have 0s appended to their ends to fill for the missing bytes).
+        /// </remarks>
+        /// <param name="begin">The iterator from which to start the known string lookup.</param>
+        /// <param name="end">The iterator pointing to the end of the input string.</param>
+        /// <param name="knownString">A reference to a pre-allocated known string, if the input matches any.</param>
+        /// <returns><c>true</c> if the input matches a known string, <c>false</c> otherwise.</returns>
         public static bool GetKnownString(this MemoryPoolIterator2 begin, MemoryPoolIterator2 end, out string knownString)
         {
             knownString = null;