Add clarification for pyyaml example

Co-authored-by: Michał Górny <mgorny@gentoo.org>

Author: zklaus

peps/pep-0780.rst

@@ -292,6 +292,15 @@ can be used::
     pyyaml-ft; "free_threaded::True" in sys_abi_info
     pyyaml; "free_threaded::True" not in sys_abi_info
 
+Note that in the absence of the features described in this PEP, early adopting
+projects have had to resort to using the fork for *all* builds of a potentially
+free threading Python interpreter; typical dependencies than look like::
+
+    pyyaml; python_version<"3.13"
+    pyyaml-ft; python_version>="3.13"
+
+which is an undesirable side effect for GIL-enabled environments.
+
 Backwards Compatibility
 =======================