docs: clarify forEachSlice callback

[dajiaohuang]

The Indexes.forEachSlice API prose referred to a rowCallback and row value, although the declaration and example expose a sliceCallback that receives a slice Id and row iterator. This updates the source documentation to match the actual API.

Fixes #304

Validation: npm run lintDocs, node --check src/@types/indexes/docs.js, and npm run compileForTest pass. npm run testDocs passes 5 of 6 documentation examples; the releases example requires a local PostgreSQL server (ECONNREFUSED on localhost:5432).