Clarify the two git-history depth knobs and how to keep indexing fast on
repos with thousands of commits:
- vex history --depth N — query-time walk, per file (caps latency)
- vex index --history-depth N — one-time build, global newest-N (like git log -nN)
Added to the setup example, the .vex.toml history comment, and the command
reference in vex.md.