Clarify income sources for free and reduced price school meals

We currently cite the SNAP income rules in https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/parameters/gov/usda/school_meals/income/sources.yaml

The free and reduced price school meals do not point to SNAP rules as far as I can tell though:
* https://www.law.cornell.edu/uscode/text/42/1758
* https://www.law.cornell.edu/cfr/text/7/245.2 provides definitions, including income which links to...
* https://www.law.cornell.edu/cfr/text/7/245.6 which doesn't define income

For items like SSI, the [USDA Eligibility Manual for School Meals](https://fns-prod.azureedge.us/sites/default/files/cn/SP36_CACFP15_SFSP11-2017a1.pdf) does include it, though the CFR may not. Let's cite that for now if it aligns, but otherwise try to find statutory or regulatory backing.