doc: add info on per-function needed feature

Document which feature, if any, is needed for a function; this should
help users in properly checking feature availability when using certain
functions.

generator/c.ml

@@ -303,6 +303,12 @@ I<The caller must free the returned buffer after use>.\n\n"
     pr "This function takes a key or passphrase parameter which
 could contain sensitive material.  Read the section
 L</KEYS AND PASSPHRASES> for more information.\n\n";
+  (match f.optional with
+  | None -> ()
+  | Some opt ->
+    pr "This function depends on the feature C<%s>.  See also
+L</guestfs_feature_available>.\n\n" opt
+  );
   (match version_added f with
   | Some version -> pr "(Added in %s)\n\n" version
   | None -> assert false

generator/fish.ml

@@ -897,9 +897,17 @@ Guestfish will prompt for these separately.\n\n";
       if f.protocol_limit_warning then
         pr "%s\n\n" protocol_limit_warning;
 
-      match deprecation_notice ~replace_underscores:true f with
+      (match deprecation_notice ~replace_underscores:true f with
       | None -> ()
       | Some txt -> pr "%s\n\n" txt
+      );
+
+      (match f.optional with
+      | None -> ()
+      | Some opt ->
+        pr "This command depends on the feature C<%s>.   See also
+L</feature-available>.\n\n" opt
+      );
   ) fishdoc_functions_sorted
 
 (* Generate documentation for guestfish-only commands. *)

generator/gobject.ml

@@ -1081,6 +1081,14 @@ guestfs_session_close (GuestfsSession *session, GError **err)
       pr " *\n";
       pr " * %s\n" doc;
 
+      (match f.optional with
+      | None -> ()
+      | Some opt ->
+        pr " * This function depends on the feature \"%s\".\n" opt;
+        pr " * See also guestfs_session_feature_available().\n";
+        pr " *\n";
+      );
+
       pr " * Returns: ";
       (match ret with
       | RErr ->

generator/java.ml

@@ -274,6 +274,13 @@ public class GuestFS {
         pr "   * %s\n" f.shortdesc;
         pr "   * </p><p>\n";
         pr "   * %s\n" doc;
+        (match f.optional with
+        | None -> ()
+        | Some opt ->
+          pr "   * </p><p>\n";
+          pr "   * This function depends on the feature \"%s\".  See also {@link #feature_available}.\n"
+            opt;
+        );
         pr "   * </p>\n";
         (match version_added f with
         | None -> ()

generator/ocaml.ml

@@ -177,6 +177,13 @@ end
       else
         pr "%s(** alias for {!%s}" indent f.name;
 
+      (match f.optional with
+      | None -> ()
+      | Some opt ->
+          has_tags := true;
+          pr "\n\n    This function depends on the feature \"%s\".  See also {!feature_available}."
+            opt
+      );
       (match f.deprecated_by with
        | None -> ()
        | Some replacement ->

generator/perl.ml

@@ -895,6 +895,12 @@ errnos:
       pr "%s\n\n" longdesc;
       if f.protocol_limit_warning then
         pr "%s\n\n" protocol_limit_warning;
+      (match f.optional with
+      | None -> ()
+      | Some opt ->
+        pr "This function depends on the feature C<%s>.  See also
+C<$g-E<gt>feature-available>.\n\n" opt
+      );
       (match deprecation_notice f with
       | None -> ()
       | Some txt -> pr "%s\n\n" txt

generator/python.ml

@@ -817,6 +817,11 @@ class GuestFS(object):
           match deprecation_notice f with
           | None -> doc
           | Some txt -> doc ^ "\n\n" ^ txt in
+        let doc =
+          match f.optional with
+          | None -> doc
+          | Some opt ->
+            doc ^ sprintf "\n\nThis function depends on the feature C<%s>.  See also C<g.feature-available>." opt in
         let doc = pod2text ~width:60 f.name doc in
         let doc = List.map (fun line -> replace_str line "\\" "\\\\") doc in
         let doc = String.concat "\n        " doc in

generator/ruby.ml

@@ -481,6 +481,11 @@ get_all_event_callbacks (guestfs_h *g, size_t *len_rtn)
           | { deprecated_by = None } -> doc
           | { deprecated_by = Some alt } ->
             doc ^ (sprintf "\n *\n * [Deprecated] In new code, use rdoc-ref:%s instead." alt) in
+        let doc =
+          match f.optional with
+          | None -> doc
+          | Some opt ->
+            doc ^ sprintf "\n *\n * [Feature] This function depends on the feature +%s+.  See also {#feature_available}[rdoc-ref:feature_available]." opt in
 
         (* Because Ruby documentation appears as C comments, we must
          * replace any instance of "/*".