Add Rhai book.

doc/src/engine/optimize/side-effects.md

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+Side-Effect Considerations for Full Optimization Level
+====================================================
+
+{{#include ../../links.md}}
+
+All of Rhai's built-in functions (and operators which are implemented as functions) are _pure_ (i.e. they do not mutate state
+nor cause any side-effects, with the exception of `print` and `debug` which are handled specially) so using
+[`OptimizationLevel::Full`] is usually quite safe _unless_ custom types and functions are registered.
+
+If custom functions are registered, they _may_ be called (or maybe not, if the calls happen to lie within a pruned code block).
+
+If custom functions are registered to overload built-in operators, they will also be called when the operators are used
+(in an `if` statement, for example) causing side-effects.
+
+Therefore, the rule-of-thumb is:
+
+* _Always_ register custom types and functions _after_ compiling scripts if [`OptimizationLevel::Full`] is used.
+
+* _DO NOT_ depend on knowledge that the functions have no side-effects, because those functions can change later on and, when that happens, existing scripts may break in subtle ways.