OData $skiptoken: add inline docs (#1217)

This should help explain how odata $skiptokens are defined and used in this codebase.

lib/util/db.js

@@ -365,6 +365,14 @@ class QueryOptions {
     return f(this.args[arg]);
   }
 
+  // Parse ODK's proprietary $skiptoken format.  From OData docs:
+  //
+  // > OData services [i.e. odk-central-backend] may use the reserved system
+  // > query option `$skiptoken` when building next links. Its content is
+  // > opaque, service-specific, and must only follow the rules for URL query
+  // > parts.
+  //
+  // See: https://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-protocol.html
   static parseSkiptoken(token) {
     const jsonString = base64ToUtf8(token.substr(2));
     return JSON.parse(jsonString);