Add missing docs for RegexRunner.Scan (#72453)

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexRunner.cs

@@ -70,6 +70,13 @@ public abstract class RegexRunner
 
         protected RegexRunner() { }
 
+        /// <summary>Used by a <see cref="Regex"/> object to scan the input <paramref name="text"/> looking for the next match.</summary>
+        /// <remarks>This API supports the product infrastructure and is not intended to be used directly from your code.</remarks>
+        /// <param name="text">The text to scan for a pattern match.</param>
+        /// <exception cref="NotSupportedException">
+        /// <see cref="ReadOnlySpan{T}"/>-based <see cref="Regex"/> methods are not supported from <see cref="Regex"/>-derived types
+        /// generated by Regex.CompileToAssembly.
+        /// </exception>
         protected internal virtual void Scan(ReadOnlySpan<char> text)
         {
             // This base implementation is overridden by all of the built-in engines and by all source-generated