Add docs

doc/stanzas/test.rst

@@ -37,6 +37,10 @@ In particular, all fields except for ``public_names`` are supported from the
 :ref:`executables stanza <shared-exe-fields>`. Alias fields apart from ``name``
 are allowed.
 
+The ``(enabled_if)`` field has special semantics: when present, it only applies
+to running the tests. The test executable is always built by default.
+If you need to restrict building the test executable, use ``(build_if)`` instead.
+
 By default, the test binaries are run without options.  The ``action`` field can
 override the test binary invocation, i.e., if you're using Alcotest and wish to
 see all the test failures on the standard output. When running Dune ``runtest``