document the new --nilseqs:on switch

changelog.md

@@ -155,7 +155,7 @@
   More details in language manual.
 
 - ``nil`` for strings/seqs is finally gone. Instead the default value for
-  these is ``"" / @[]``.
+  these is ``"" / @[]``. Use ``--nilseqs:on`` for a transition period.
 
 - Accessing the binary zero terminator in Nim's native strings
   is now invalid. Internally a Nim string still has the trailing zero for

doc/advopt.txt

@@ -68,6 +68,8 @@ Advanced options:
   --oldNewlines:on|off      turn on|off the old behaviour of "\n"
   --laxStrings:on|off       when turned on, accessing the zero terminator in
                             strings is allowed; only for backwards compatibility
+  --nilseqs:on|off          allow 'nil' for strings/seqs for
+                            backwards compatibility
   --skipCfg                 do not read the general configuration file
   --skipUserCfg             do not read the user's configuration file
   --skipParentCfg           do not read the parent dirs' configuration files