manual: document builtins.functionArgs

The text is just a conversion of comment from src/libexpr/primops.cc

doc/manual/expressions/builtins.xml

@@ -348,6 +348,24 @@ stdenv.mkDerivation {
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.functionArgs</function>
+    <replaceable>f</replaceable></term>
+
+    <listitem><para>
+    Return a set containing the names of the formal arguments expected
+    by the function <replaceable>f</replaceable>.
+    The value of each attribute is a Boolean denoting whether the corresponding
+    argument has a default value.  For instance,
+    <literal>functionArgs ({ x, y ? 123}: ...)  =  { x = false; y = true; }</literal>.
+    </para>
+
+    <para>"Formal argument" here refers to the attributes pattern-matched by
+    the function.  Plain lambdas are not included, e.g.
+    <literal>functionArgs (x: ...)  =  { }</literal>.
+    </para></listitem>
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.fromJSON</function> <replaceable>e</replaceable></term>
 
     <listitem><para>Convert a JSON string to a Nix

src/libexpr/primops.cc

@@ -1154,7 +1154,7 @@ static void prim_catAttrs(EvalState & state, const Pos & pos, Value * * args, Va
 
 /* Return a set containing the names of the formal arguments expected
    by the function `f'.  The value of each attribute is a Boolean
-   denoting whether has a default value.  For instance,
+   denoting whether the corresponding argument has a default value.  For instance,
 
       functionArgs ({ x, y ? 123}: ...)
    => { x = false; y = true; }