Doc consistency pass in next dev cycle

[ihnorton]

@stavrospapadopoulos is planning to do a full pass on the docs in the next development cycle to improve consistency, including:

• further clarification of the capacity definition (ref #1167).
• change the parameter from capacity -> sparse_capacity
• per @jakebolewski, set_capacity needs to return the dense capacity (e.g. for a HL api iterator over tiles)