Previously, there were three different DTDs used. Let's switch to a single one.
We will go with 4.5, since it is latest version that does not have any backwards incompatible changes.
The --verbose and --ostree-verbose options are global to all
subcommands, but --version can only be used with the main "flatpak"
command, so fix the man pages to reflect that.
We no longer run appstream-builder (see commit 455d3a7b2) so update the
documentation for the --update-appstream option.
Closes: #1360
Approved by: alexlarsson