vendor._gowin: Document SDC parsing quirk

amaranth/vendor/_gowin.py

@@ -437,6 +437,8 @@ def _osc_div(self):
             file delete -force {{name}}.fs
             file copy -force impl/pnr/project.fs {{name}}.fs
         """,
+        # Gowin is using neither Tcl nor the Synopsys code to parse SDC files, so the grammar
+        # deviates from the usual (eg. no quotes, no nested braces).
         "{{name}}.sdc": r"""
         // {{autogenerated}}
         {% for signal, frequency in platform.iter_signal_clock_constraints() -%}