Add Rhai book.

doc/src/engine/optimize/volatility.md

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+Volatility Considerations for Full Optimization Level
+===================================================
+
+{{#include ../../links.md}}
+
+Even if a custom function does not mutate state nor cause side-effects, it may still be _volatile_,
+i.e. it _depends_ on the external environment and is not _pure_.
+
+A perfect example is a function that gets the current time - obviously each run will return a different value!
+
+The optimizer, when using [`OptimizationLevel::Full`], will _merrily assume_ that all functions are _pure_,
+so when it finds constant arguments (or none) it eagerly executes the function call and replaces it with the result.
+
+This causes the script to behave differently from the intended semantics.
+
+Therefore, **avoid using [`OptimizationLevel::Full`]** if non-_pure_ custom types and/or functions are involved.