Improve package's documentation

[bonjourmauko]

Linked to #1002

Context:

Documentation helps reusers, and interested public in general, to quickly understand OpenFisca.

Complications:

Documentation is either broken, outdated, and/or fairly incomplete.

TODO

• Add docs to commons #1251
• Add docs to data storage #1262
• Add docs to entities #1252
• Add docs to experimental #1274
• Add docs to holders #1270
• Add docs to enums #1271
• 6. Improve openfisca_core.parameters documentation
• 7. Improve openfisca_core.periods documentation
• Fix populations documentation #1277
• 9. Improve openfisca_core.projectors documentation
• 10. Improve openfisca_core.reforms documentation
• 11. Improve openfisca_core.simulation_builder documentation
• 12. Improve openfisca_core.simulations documentation
• 13. Improve openfisca_core.taxbenefitsystems documentation
• 14. Improve openfisca_core.taxscales documentation
• 15. Improve openfisca_core.tracers documentation
• 16. Improve openfisca_core.variables documentation
• 17. Improve openfisca_web_api.loader documentation