Issue #3616580: Say what an empty Places held does, and how it differs from 0

Reword the three Places held descriptions so each says what its own empty value does, and what 0 does beside it.

• resource and preset: empty withholds no number, which lets the places pinned to the allotment be all it holds; 0 holds nothing at all, pinned places included
• session: empty keeps what the resource sets aside; 0 holds nothing on this session

No behavior change. The French translation moves with the strings in the same commit.