Document the --code-highlight default (#9883)

Also normalized all help text using the patterns:

* `One sentence help text`
* `First sentence of help. Second sentence of help.`

src/_pytest/fixtures.py

@@ -1352,7 +1352,7 @@ def pytest_addoption(parser: Parser) -> None:
         "usefixtures",
         type="args",
         default=[],
-        help="list of default fixtures to be used with this project",
+        help="List of default fixtures to be used with this project",
     )