Document tmt export posibilities

[lukaszachy]

tmt [test|plan|story] export --help doesn't list which options are avaialble, once sees that only if the --how contains invalid value.

Also we don't document the possibilities at one place in the documentation. polarion/nitrate are mentioned, but dict, yaml (and new json) are nowhere listed.

[happz]

Help listing the supported formats is part of #2059, https://github.com/teemtee/tmt/pull/2059/files#diff-2bcd3056539a32d0370d652cfc6c04bd19b674cae84c341c3be7fc51cc3c60e6R595

Should be a stand-alone PR, though, it's how my quest after click.option wrapper started, but it does not depend on #2059.