Document which helper functions work with which api functions

I think some standard notation about which "Helper" functions (mostly the filters) work with which "API" functions (as sources of data).

Currently this is only conveyed as part of the freeform description of the functions (usually the filter functions).

I'm leaning towards some sort of standard header/notations for each function. Though a table might also work.

No files, tests, or entry points are named. Start by inventorying the Helper and API functions and reviewing their existing freeform descriptions, then choose and document a consistent notation or table; done means the supported relationships are recorded consistently for each function.

Written by the indexing model from the issue text.