Clarify Enumerable.Prepend returns a new sequence

Changed the summary from "Adds a value to the beginning of the
  sequence" to "Returns a new sequence that begins with the specified
  element followed by the elements of the source sequence."

  The previous wording incorrectly implied in-place modification of
  the original sequence. The new wording makes it clear that Prepend
  returns a new sequence without modifying the source, consistent with
  LINQ's deferred execution and immutability principles.

Author: oii-nasif

xml/System.Linq/Enumerable.xml

@@ -11681,7 +11681,7 @@ If comparer is `null`, the default comparer <see cref="P:System.Collections.Gene
         <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
         <param name="source">A sequence of values.</param>
         <param name="element">The value to prepend to <paramref name="source" />.</param>
-        <summary>Adds a value to the beginning of the sequence.</summary>
+        <summary>Returns a new sequence that begins with the specified element followed by the elements of the source sequence.</summary>
         <returns>A new sequence that begins with <paramref name="element" />.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[