Implement warning for unmatched test specs

docs/command-line.md

@@ -212,14 +212,22 @@ This option transforms tabs and newline characters into ```\t``` and ```\n``` re
 ## Warnings
 <pre>-w, --warn &lt;warning name></pre>
 
-Enables reporting of suspicious test runs. There is currently only one
-available warning.
+You can think of Catch2's warnings as the equivalent of `-Werror` (`/WX`)
+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.
+
+There are currently two warnings implemented:
 
 ```
-    NoAssertions   // Fail test case / leaf section if no assertions
-                   // (e.g. `REQUIRE`) is encountered.
+    NoAssertions        // Fail test case / leaf section if no assertions
+                        // (e.g. `REQUIRE`) is encountered.
+    UnmatchedTestSpec   // Fail test run if any of the CLI test specs did
+                        // not match any tests.
 ```
 
+> `UnmatchedTestSpec` was introduced in Catch2 X.Y.Z.
+
 
 <a id="reporting-timings"></a>
 ## Reporting timings

docs/release-notes.md

@@ -173,6 +173,7 @@ new design.
   * When deferred tu runtime, it behaves like `CHECK`, and not like `REQUIRE`.
 * 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
 
 
 ### Fixes