In 3.x the `perPage` option on piece types should be renamed `apiPerPage`

[abea]

The problem to solve

Both piece types and piece page types have a perPage option in 3.x. The module types are related and the properties have identical keys, but they have different purposes. We could make those more clear by renaming one.

Proposed solution

In piece page types, perPage defines the number of pieces displayed on index pages of that type. For piece type modules, perPage defines the number of pieces included in each "page" of API GET requests.

We should rename the option on piece type modules to apiPerPage to make its functionality clearer. We could cover the BC break by letting perPage serve as an undocumented alias.

Alternatives

We do nothing and count on documentation and the fact that it probably won't be used all that often for piece types anyway.

[boutell]

perPage is used pretty heavily I would expect, since it has a default and it might not be what is wanted all the time.

apiPerPage seems reasonable to me.