Add Rhai book.

doc/src/safety/max-call-stack.md

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+Maximum Call Stack Depth
+=======================
+
+{{#include ../links.md}}
+
+Limiting How Stack Usage by Scripts
+----------------------------------
+
+Rhai by default limits function calls to a maximum depth of 128 levels (16 levels in debug build).
+
+This limit may be changed via the `Engine::set_max_call_levels` method.
+
+A script exceeding the maximum call stack depth will terminate with an error result.
+
+This check can be disabled via the [`unchecked`] feature for higher performance (but higher risks as well).
+
+```rust
+let mut engine = Engine::new();
+
+engine.set_max_call_levels(10); // allow only up to 10 levels of function calls
+
+engine.set_max_call_levels(0);  // allow no function calls at all (max depth = zero)
+```
+
+
+Setting Maximum Stack Depth
+--------------------------
+
+When setting this limit, care must be also taken to the evaluation depth of each _statement_
+within a function. It is entirely possible for a malicious script to embed a recursive call deep
+inside a nested expression or statement block (see [maximum statement depth]).