Refactor `spec-ref` kw's in `processor.clj` for better consistency and phrasing

[stanislowskij]

The keywords here are very messy and inconsistent. Examples:

• Multiple different keywords that translate to "a sequence":

:collection "a sequence" :coll "a sequence" :collection-map "a sequence"

• Hyper-specific translation for the actual :sequence keyword:

:sequence "a sequence of vectors with only 2 elements or a map with key-value pairs"'

• Any other examples of inconsistent keywording for specs

Some of these should definitely be revisited, not only here, but in our core function specs.

• Also need notes in a possible CONTRIBUTING.md document about how to write these specs using the correct keywords. See: [Le #150] Improve documentation and code organization in core files #60