Update docs about case-insensitive expression matching

doc/en/example/markers.rst

@@ -148,6 +148,10 @@ which implements a substring match on the test names instead of the
 exact match on markers that ``-m`` provides.  This makes it easy to
 select tests based on their names:
 
+.. versionadded: 5.3.1/6.0
+
+The expression matching is now case-insensitive.
+
 .. code-block:: pytest
 
     $ pytest -v -k http  # running with the above defined example module

doc/en/usage.rst

@@ -94,8 +94,8 @@ Pytest supports several ways to run and select tests from the command-line.
 
     pytest -k "MyClass and not method"
 
-This will run tests which contain names that match the given *string expression*, which can
+This will run tests which contain names that match the given *string expression* (case-insensitive),
-include Python operators that use filenames, class names and function names as variables.
+which can include Python operators that use filenames, class names and function names as variables.
 The example above will run ``TestMyClass.test_something``  but not ``TestMyClass.test_method_simple``.
 
 .. _nodeids:

src/_pytest/mark/__init__.py

@@ -52,7 +52,8 @@ def pytest_addoption(parser):
         "-k 'not test_method and not test_other' will eliminate the matches. "
         "Additionally keywords are matched to classes and functions "
         "containing extra names in their 'extra_keyword_matches' set, "
-        "as well as functions which have names assigned directly to them.",
+        "as well as functions which have names assigned directly to them. "
+        "The matching is case-insensitive.",
     )
 
     group._addoption(