Add comment above the %soname_version definition

pypy3.9.spec

@@ -447,6 +447,8 @@ BuildPyPy \
 %global installation_archive_name pypy%{pyversion}-%{version_}
 %global packaging_builddir builddir
 %global packaged_prefix %{packaging_builddir}/%{installation_archive_name}
+# We will set an arbitrary downstream-only soname version, as it is required
+# See https://docs.fedoraproject.org/en-US/packaging-guidelines/#_downstream_so_name_versioning
 %global soname_version 0.1
 
 %{bootstrap_python_interp} pypy/tool/release/package.py --archive-name '%{installation_archive_name}' --builddir '%{packaging_builddir}' --no-embedded-dependencies