docs: add entry about SYNTH_HIER_SEPARATOR

Author: magancarz

flow/scripts/variables.yaml

@@ -677,6 +677,10 @@ SYNTH_ARGS:
   description: |
     Optional synthesis variables for yosys.
   default: -flatten
+SYNTH_HIER_SEPARATOR:
+  description: |
+    Separator used for the synthesis flatten stage.
+  default: .
 VERILOG_TOP_PARAMS:
   description: |
     Apply toplevel params (if exist).