docs: clarify request depth semantics

[StantonMatt]

Summary

• Document Request.Depth and MaxDepth as 1-based values.
• Update the max-depth examples so their comments match root depth 1 and child depth 2.
• Add a regression test for root and child request depth values.

Fixes #769

Verification

• go test . -run TestCollectorRequestDepthValues -count=1 -v
• go test ./... -count=1
• go test ./_examples/max_depth ./_examples/parallel
• git diff --check