Improved docs on BeLowerCased and BeUpperCased for mixed strings.

Src/FluentAssertions/Primitives/StringAssertions.cs

@@ -1285,6 +1285,11 @@ public AndConstraint<TAssertions> BeNullOrWhiteSpace(string because = "", params
         /// <summary>
         /// Asserts that all characters in a string are in upper casing.
         /// </summary>
+        /// <remarks>
+        /// Be careful that numbers and special characters don't have casing, so  <see cref="BeUpperCased"/>
+        /// will always fail on a string that contains anything but alphabetic characters.
+        /// In those cases, we recommend using <see cref="NotBeLowerCased"/>.
+        /// </remarks>
         /// <param name="because">
         /// A formatted phrase as is supported by <see cref="string.Format(string,object[])" /> explaining why the assertion
         /// is needed. If the phrase does not start with the word <i>because</i>, it is prepended automatically.
@@ -1325,6 +1330,11 @@ public AndConstraint<TAssertions> NotBeUpperCased(string because = "", params ob
         /// <summary>
         /// Asserts that all characters in a string are in lower casing.
         /// </summary>
+        /// <remarks>
+        /// Be careful that numbers and special characters don't have casing, so <see cref="BeLowerCased"/> will always fail on
+        /// a string that contains anything but alphabetic characters.
+        /// In those cases, we recommend using <see cref="NotBeUpperCased"/>.
+        /// </remarks>
         /// <param name="because">
         /// A formatted phrase as is supported by <see cref="string.Format(string,object[])" /> explaining why the assertion
         /// is needed. If the phrase does not start with the word <i>because</i>, it is prepended automatically.

docs/_pages/releases.md

@@ -15,6 +15,8 @@ sidebar:
 
 ### Fixes
 
+* Improved the documentation on `BeLowerCased` and `BeUpperCased` for strings with non-alphabetic characters - [#0000](https://github.com/fluentassertions/fluentassertions/pull/1791)
+
 ## 6.4.0
 
 ### What's New

docs/_pages/strings.md

@@ -18,12 +18,19 @@ theString.Should().NotBeEmpty("because the string is not empty");
 theString.Should().HaveLength(0);
 theString.Should().BeNullOrWhiteSpace(); // either null, empty or whitespace only
 theString.Should().NotBeNullOrWhiteSpace();
+```
+
+To ensure the characters in a string are all (not) upper or lower cased, you can use the following assertions.
+
+```csharp
 theString.Should().BeUpperCased();
 theString.Should().NotBeUpperCased();
 theString.Should().BeLowerCased();
 theString.Should().NotBeLowerCased();
 ```
 
+However, be careful that numbers and special characters don't have casing, so `BeUpperCased` and `BeLowerCased` will always fail on a string that contains anything but alphabetic characters. In those cases, we recommend using `NotBeUpperCased` or `NotBeLowerCased`.
+
 Obviously you’ll find all the methods you would expect for string assertions.
 
 ```csharp