docs: order of tags can change

[tbm]

Meta-data is not ordered in beancount.

Test:

2020-01-01 open Assets:Test

2022-12-26 * "Test order of tags"
    #foo
    #bar
    Assets:Test         10.00 EUR
    Assets:Test        -10.00 EUR

Output:

2022-12-26 * Test order of tags
    ; :bar:foo:
    Assets:Test                                                   10.00 EUR
    Assets:Test                                                  -10.00 EUR

This is a known issue, so needs to be documented.