docs: docuent identifiers.json

Author: crhntr

specification/README.md

@@ -7,7 +7,7 @@ For now the code in ../query is implemented by hand.
 The current specifications are layed out as follows:
 - query_parameters.json: many of the parameters are shared across query functions. Specifying them here allows a single source of truth for what a "symbol" is
 - function/*.json: each of these files contains a list of function=X specifications and the additional query parameters supported by X
-- identifiers.json: maps string literals from the API (column names, query parameter keys or query parameter values) to Go identifiers
+- identifiers.json: maps string literals from the API (column names, query parameter keys or query parameter values) to Go identifiers. The first value is a public identifier the second is a private identifier.
 
 
 The Go files specification.go and specification_test.go help with structural and referential integrity of the JSON files.