Modify rule S2223: Update description to include also the case of global state (#1583)

rules/S2223/csharp/rule.adoc

@@ -1,4 +1,11 @@
-A ``++static++`` field that is neither constant nor read-only is not thread-safe. Correctly accessing these fields from different threads needs synchronization with ``++lock++``s. Improper synchronization may lead to unexpected results, thus publicly visible static fields are best suited for storing non-changing data shared by many consumers. To enforce this intent, these fields should be marked ``++readonly++`` or converted to constants.
+Non-private `static` fields that are neither `const` nor `readonly` can lead to errors and unpredictable behavior.
+
+This can happen because:
+* Any object can modify these fields and alter the global state. This makes the code more difficult to read, debug and test.
+* Correctly accessing these fields from different threads needs synchronization with `lock`. Improper synchronization may lead to unexpected results.
+
+Publicly visible static fields should only be used to store shared data that does not change.
+To enforce this intent, these fields should be marked `readonly` or converted to `const`.
 
 
 == Noncompliant Code Example
@@ -8,7 +15,7 @@ A ``++static++`` field that is neither constant nor read-only is not thread-safe
 public class Math
 {
   public static double Pi = 3.14;  // Noncompliant
-} 
+}
 ----
 or
 
@@ -20,8 +27,8 @@ public class Shape
 
   private class EmptyShape : Shape
   {
-  } 
-} 
+  }
+}
 ----
 
 
@@ -32,7 +39,7 @@ public class Shape
 public class Math
 {
   public const double Pi = 3.14;
-} 
+}
 ----
 or
 
@@ -40,12 +47,12 @@ or
 ----
 public class Shape
 {
-  public static readonly Shape Empty = new EmptyShape(); 
+  public static readonly Shape Empty = new EmptyShape();
 
   private class EmptyShape : Shape
   {
-  } 
-} 
+  }
+}
 ----