The syntax for the Expression Language is entirely documented within the Ignition user manual, as this is entirely defined by Inductive Automation.
On top of the basic syntax and operators, expressions may use many functions, which are documented in an appendix, and can be supplemented by functions from third party modules. Like mine, as an example.
Python and Jython are not documented, except briefly and in examples, in Ignition's User manual, as these are open source components with large and thorough public documentation.