Add two missing since-annotations to user's guide

[skip ci]

Cabal/doc/developing-packages.rst

@@ -2539,6 +2539,7 @@ field of the stanza that is using it, besides :pkg-field:`other-modules` or
 complex build hooks for this poweruser case.
 
 .. pkg-field:: autogen-modules: module list
+   :since: 2.0
 
    .. TODO: document autogen-modules field
 

Cabal/doc/nix-local-build.rst

@@ -774,6 +774,7 @@ The following settings control the behavior of the dependency solver:
 .. cfg-field:: allow-older: none, all, list of scoped package names (space or comma separated)
                --allow-older, --allow-older=[none,all,[scope:][^]pkg]
     :synopsis: Lift dependency lower bound constaints.
+    :since: 2.0
 
     :default: ``none``