The best documentation that I know of for this is the source code, which defines the JSON encoding for ScriptData.
The tags "int" and "bytes" encode those primitives. The tag "list" encodes [ScriptData] and the tag "map" encodes [(ScriptData,ScriptData)]. The "constructor" tag with a "fields" tag encodes Haskell constructors, where the integer following "constructor" is the zero-based index to the constructor that is being used; the fields are encoded in order. Use the template Haskell PlutusTx.makeIsDataIndexed or PlutusTx.unsafeMakeIsData to create the encoding for a custom data type.
For example, for the type data Example = This Integer | That, the value This 5 becomes {"constructor": 0, "fields": [{"int": 5}]} and That becomes {"constructor": 1, "fields": []}.