Document System.IO.Enumeration.FileSystemEnumerable (#3413)

* Document System.IO.Enumeration.FileSystemEnumerable

* minor edits

* suggestions by mairaw

* edits

* remove internal implementation detail

Author: carlossanlop

xml/System.IO.Enumeration/FileSystemEnumerable`1.xml

@@ -54,11 +54,17 @@
         <Parameter Name="options" Type="System.IO.EnumerationOptions" />
       </Parameters>
       <Docs>
-        <param name="directory">To be added.</param>
-        <param name="transform">To be added.</param>
-        <param name="options">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="directory">The path of the directory where the enumeration will be performed.</param>
+        <param name="transform">A delegate method for transforming raw find data into a result.</param>
+        <param name="options">An object describing the enumeration options.</param>
+        <summary>Initializes a new instance of the <see cref="T:System.IO.Enumeration.FileSystemEnumerable`1" /> class with specific search and filtering options.</summary>
+         <remarks>To be added.</remarks>
+         <exception cref="T:System.ArgumentNullException"><paramref name="directory" /> or <paramref name="transform" /> is <see langword="null" />.</exception>
+         <exception cref="T:System.ArgumentException">The <paramref name="directory" /> path is empty.
+
+-or-
+
+<paramref name="directory" /> contains a null character "\0".</exception>
       </Docs>
     </Member>
     <Member MemberName="GetEnumerator">