Move constancy into Dynamic.

doc/src/engine/optimize/index.md

@@ -64,6 +64,15 @@ are spliced into the script text in order to turn on/off certain sections.
 For fixed script texts, the constant values can be provided in a user-defined [`Scope`] object
 to the [`Engine`] for use in compilation and evaluation.
 
+### Caveat
+
+If the [constants] are modified later on (yes, it is possible, via Rust functions),
+the modified values will not show up in the optimized script.
+Only the initialization values of [constants] are ever retained.
+
+This is almost never a problem because real-world scripts seldom modify a constant,
+but the possibility is always there.
+
 
 Eager Operator Evaluations
 -------------------------

doc/src/engine/optimize/optimize-levels.md

@@ -11,6 +11,14 @@ There are three levels of optimization: `None`, `Simple` and `Full`.
   (i.e. it only relies on static analysis and [built-in operators] for constant [standard types],
   and will not perform any external function calls).
 
+  However, it is important to bear in mind that _constants propagation_ is performed with the
+  caveat that, if [constants] are modified later on (yes, it is possible, via Rust functions),
+  the modified values will not show up in the optimized script.  Only the initialization values
+  of [constants] are ever retained.
+
+  Furthermore, overriding a [built-in operator][built-in operators] in the [`Engine`] afterwards
+  has no effect after the optimizer replaces an expression with its calculated value.
+
 * `Full` is _much_ more aggressive, _including_ calling external functions on constant arguments to determine their result.
   One benefit to this is that many more optimization opportunities arise, especially with regards to comparison operators.