There were two reasons why no docs for this class appeared in the HTML
documentation:
* flatpak-transaction-operation.xml was not included by
flatpak-docs.xml.
* all its symbols were listed in <SUBSECTION Standard>, which is hidden
from the HTML output. It appears that if a section has no visible
symbols, it's omitted.
Many symbols which belong to FlatpakTransaction were erroneously grouped
with FlatpakTransactionOperation and so also hidden; fix this too.
Closes: #1995
Approved by: alexlarsson