Document idempotence of D-Bus API in design docs

[jbaublitz]

This is mentioned in the D-Bus docs but not in the design docs.

[mulkieran]

This should probably be done with stratis-storage/project#186 so that we have clearly described exactly what we are achieving.

[mulkieran]

taking this off any particular calendar, we may move from LyX to LaTeX first.

[jbaublitz]

I will be taking a look at this along with my other documentation updates like stratis-docs#243.