JSON is no longer strictly valid JSON

[ptheywood]

Since FLAMEGPU/FLAMEGPU2#969, the json produced by the relevant FLAME GPU api calls is not strictly valid JSON, due to the inclusion of nan/inf like values, as produced by RapidJSON.

This should be documented in the relavant areas, potentially linking to the relevant rapidJSON docs.

It may be valid JSON5 / a common extension though.

(Note prior to this change it was also wrong, due to the absence of data if NaN).