tests: returning few comments dropped during TMT conversion

I find these comments useful, especially for my future self who would be
dealing with the subtle changes between Fedora and downstream.

tests/integration-test-suite/main.fmf

@@ -1,4 +1,8 @@
 summary: Run the upstream LLVM integration test suite
+description: |
+  Run the upstream LLVM integration test suite
+
+  NOTE: don't forget to disable libcxx tests when libcxx is not available, by adding -DENABLE_LIBCXX=OFF.
 test: ./test.sh
 require:
   - cmake
@@ -8,12 +12,17 @@ require:
   - clang-tools-extra
   - compiler-rt
   - ninja-build
-  - libcxx-devel
   - libomp-devel
-  - python-lit
   - lld
   - lldb
   - git
   - make
   - libstdc++-static
   - clang-devel
+
+  # In Fedora, python-lit is valid. In RHEL, however, the actual package is python3-lit.
+  - python-lit
+  # - python3-lit
+
+  # Required in Fedora, but not shipped with RHEL. For future reference, comment the libcxx out as needed.
+  - libcxx-devel

tests/rust-sanity/main.fmf

@@ -1,5 +1,16 @@
 summary: Test basic cargo usage
 test: ./test.sh
 require:
+# Required for rust and possibly other dependant packages
+#
+# NOTE #1: drop the requirement once the transitional period between releases is gone
+# and llvm-compat is no longer a) needed, and b) part of the build.
+#
+# NOTE #2: keep it the first requirement. Depending on the order of requirements, placing it somewhere
+# else in the list may lead to harness uninstalling and re-installing LLVM packages. That a) would be
+# a waste of time, b) could potentially hide issues, leading to testing unexpected NVRs.
+#
+#  - llvm-compat
+
   - cargo
   - rust