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/setuponly.py

@@ -18,13 +18,13 @@ def pytest_addoption(parser: Parser) -> None:
         "--setuponly",
         "--setup-only",
         action="store_true",
-        help="only setup fixtures, do not execute tests.",
+        help="Only setup fixtures, do not execute tests",
     )
     group.addoption(
         "--setupshow",
         "--setup-show",
         action="store_true",
-        help="show setup of fixtures while executing tests.",
+        help="Show setup of fixtures while executing tests",
     )