Modify rule S2970: Improve description of supported test library func…

…tions (#1518)

rules/S2970/csharp/rule.adoc

@@ -1,19 +1,35 @@
-include::../description.adoc[]
+It is very easy to write incomplete assertions when using some test frameworks. This rule enforces complete assertions in the following cases:
+
+* Fluent Assertions: https://fluentassertions.com/introduction[`Should()`] is not followed by an assertion invocation.
+* NFluent: https://www.n-fluent.net[`Check.That()`] is not followed by an assertion invocation.
+* NSubstitute: https://nsubstitute.github.io/help/received-calls[`Received()`] is not followed by an invocation.
+
+In such cases, what is intended to be a test doesn't actually verify anything.
 
 == Noncompliant Code Example
 
 [source,csharp]
 ----
 string actual = "Hello World!";
-actual.Should();  // Noncompliant
+// Fluent Assertions
+actual.Should();     // Noncompliant
+// NFluent
+Check.That(actual);  // Noncompliant
+// NSubstitute
+command.Received();  // Noncompliant
 ----
 
 == Compliant Solution
 
 [source,csharp]
 ----
 string actual = "Hello World!";
+// Fluent Assertions
 actual.Should().Contain("Hello");
+// NFluent
+Check.That(actual).Contains("Hello");
+// NSubstitute
+command.Received().Execute();
 ----
 
 ifdef::env-github,rspecator-view[]