Document fromTOML, hasContext and getContext builtins

Until now, these functions were completely missing in the Nix manual.

src/libexpr/primops/context.cc

@@ -28,7 +28,12 @@ static void prim_hasContext(EvalState & state, const PosIdx pos, Value * * args,
 
 static RegisterPrimOp primop_hasContext({
     .name = "__hasContext",
-    .arity = 1,
+    .args = {"s"},
+    .doc = R"(
+      Return `true` if string *s* has a non-empty context. The
+      context can be obtained with
+      [`getContext`](#builtins-getContext).
+    )",
     .fun = prim_hasContext
 });
 
@@ -133,7 +138,22 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
 
 static RegisterPrimOp primop_getContext({
     .name = "__getContext",
-    .arity = 1,
+    .args = {"s"},
+    .doc = R"(
+      Return the context associated with string *s*.
+
+      For example,
+
+      ```nix
+      builtins.getContext "X ${derivation { name = "a"; builder = "b"; system = "c"; }}"
+      ```
+
+      evaluates to
+
+      ```
+      { "/nix/store/arhvjaf6zmlyn8vh8fgn55rpwnxq0n7l-a.drv" = { outputs = [ "out" ]; }; }
+      ```
+    )",
     .fun = prim_getContext
 });
 

src/libexpr/primops/fromTOML.cc

@@ -92,7 +92,21 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
 
 static RegisterPrimOp primop_fromTOML({
     .name = "fromTOML",
-    .arity = 1,
+    .args = {"e"},
+    .doc = R"(
+      Convert a TOML string to a Nix value. For example,
+
+      ```nix
+      builtins.fromTOML ''
+        x=1
+        s="a"
+        [table]
+        y=2
+      ''
+      ```
+
+      returns the value `{ s = "a"; table = { y = 2; }; x = 1; }`.
+    )",
     .fun = prim_fromTOML
 });