#436 Document filter(po): plural as comment in japanese target

[Jean-Christophe Helary]

https://github.com/omegat-org/omegat/pull/1165

Japanese is nplural=1 language.
When there is no marker in singular msgid, translator can not see plural form of msgid when translate in OmegaT panes. This adds plural form of msgid as a comment of a segment which is a singular form.