Clarify usage of string interpolation in LINQ Data Transformation (#18764)

* clarify 'performing operations on source elements'
- refactor query description to clearly describe steps in operation
- add brief explanation of string interpolation and link to
string interpolation page

* update code snippet
- change output sequence variable name from 'query' to 'output'
- add linq example using method syntax in addition to query syntax
- add allman braces to foreach loop printing results

* add extra sentence to include concatenation

Author: jmarkman

docs/csharp/programming-guide/concepts/linq/data-transformations-with-linq.md

@@ -84,7 +84,11 @@ Language-Integrated Query (LINQ) is not only about retrieving data. It is also a
  For more information, see [Creating XML Trees in C# (LINQ to XML)](./creating-xml-trees-linq-to-xml-2.md).  
 
 ## Performing Operations on Source Elements  
- An output sequence might not contain any elements or element properties from the source sequence. The output might instead be a sequence of values that is computed by using the source elements as input arguments. The following simple query, when it is executed, outputs a sequence of strings whose values represent a calculation based on the source sequence of elements of type `double`.  
+ An output sequence might not contain any elements or element properties from the source sequence. The output might instead be a sequence of values that is computed by using the source elements as input arguments.
+
+ The following query will take a sequence of numbers that represent radii of circles, calculate the area for each radius, and return an output sequence containing strings formatted with the calculated area.
+
+ Each string for the output sequence will be formatted using [string interpolation](../../../language-reference/tokens/interpolated.md). An interpolated string will have a `$` in front of the string's opening quotation mark, and operations can be performed within curly braces placed inside the interpolated string. Once those operations are performed, the results will be concatenated.
 
 > [!NOTE]
 > Calling methods in query expressions is not supported if the query will be translated into some other domain. For example, you cannot call an ordinary C# method in [!INCLUDE[vbtecdlinq](~/includes/vbtecdlinq-md.md)] because SQL Server has no context for it. However, you can map stored procedures to methods and call those. For more information, see [Stored Procedures](../../../../framework/data/adonet/sql/linq/stored-procedures.md).  

samples/snippets/csharp/VS_Snippets_VBCSharp/CsLINQGettingStarted/CS/Class1.cs

@@ -252,15 +252,22 @@ static void Main()
             // Data source.
             double[] radii = { 1, 2, 3 };
 
-            // Query.
-            IEnumerable<string> query =
+            // LINQ query using method syntax.
+            IEnumerable<string> output = 
+                radii.Select(r => $"Area for a circle with a radius of '{r}' = {r * r * Math.PI:F2}");
+
+            /*
+            // LINQ query using query syntax.
+            IEnumerable<string> output =
                 from rad in radii
                 select $"Area = {rad * rad * Math.PI:F2}";
+            */
 
-            // Query execution.
-            foreach (string s in query)
+            foreach (string s in output)
+            {
                 Console.WriteLine(s);
-
+            }
+                
             // Keep the console open in debug mode.
             Console.WriteLine("Press any key to exit.");
             Console.ReadKey();