docs: Get API docs working again

This changes around a few things that didn't work for me:
* Section names seem to be ostree-* instead of libostree-*
* Also XML files are ostree-* (they didn't show up at all)
- gtk-doc doesn't seem to parse const _OSTREE_PUBLIC correctly
* pull documentation is now on the actual functions rather than stubs
* Update gitignore with some more files

And there some changes to make gtk-doc give fewer warnings (not finished)

Closes: #327
Approved by: cgwalters

src/libostree/ostree-diff.c

@@ -210,6 +210,8 @@ diff_add_dir_recurse (GFile          *d,
  * @modified: (element-type OstreeDiffItem): Modified files
  * @removed: (element-type Gio.File): Removed files
  * @added: (element-type Gio.File): Added files
+ * @cancellable: Cancellable
+ * @error: Error
  *
  * Compute the difference between directory @a and @b as 3 separate
  * sets of #OstreeDiffItem in @modified, @removed, and @added.