Allow enabling multiple warnings in one invocation

docs/command-line.md

@@ -217,6 +217,8 @@ flag for C++ compilers. It turns some suspicious occurences, like a section
 without assertions, into errors. Because these might be intended, warnings
 are not enabled by default, but user can opt in.
 
+You can enable multiple warnings at the same time.
+
 There are currently two warnings implemented:
 
 ```

docs/release-notes.md

@@ -174,6 +174,7 @@ new design.
 * You can have multiple tests with the same name, as long as other parts of the test identity differ (#1915, #1999, #2175)
   * Test identity includes test's name, test's tags and and test's class name if applicable.
 * Added new warning, `UnmatchedTestSpec`, to error on test specs with no matching tests
+* The `-w`, `--warn` warning flags can now be provided multiple times to enable multiple warnings
 
 
 ### Fixes