Problem

The statistics configuration accepts all, fast, none, clear, cache_walk, tree_walk and, on cursors, size. The generated config text and statistics.dox describe only the first four. Per-page btree statistics say "only reported if tree_walk or all statistics are enabled", but nothing explains what tree_walk is or costs.

Behaviour

• all enables fast, cache_walk and tree_walk.
• tree_walk reads every page in the tree for per-page btree counters. Cost scales with tree size.
• cache_walk walks pages in cache for per-page cache statistics. Cost scales with cache size.
• size is cursor-only, returns on-disk sizes, and excludes all, fast and clear.

Change

Add one sentence per mode to the statistics descriptions in api_data.py, regenerate, and add a paragraph to statistics.dox. Check with the team whether the internal "implies fast" behaviour should be documented.