Updated docs

doc/CompilationStages.md

@@ -17,10 +17,11 @@ flowchart LR
     subgraph GDScript
       direction TB
       Bind[Bind all top-level declarations in a global symbol table]
+      Const[Check that any expressions in constant position are actually constant]
       Compile[Compile IR into GDScript syntax representation]
       Optimize[Run GDScript-Level Optimizations]
       Output[Output GDScript to .gd file]
-      Bind-->Compile-->Optimize-->Output
+      Bind-->Const-->Compile-->Optimize-->Output
     end
     AST-->IR-->GDScript
 ```
@@ -61,6 +62,11 @@ top-level declarations have their names bound in the global symbol
 table. This ensures that, during compilation, functions can access
 functions declared anywhere in the file.
 
+With the symbol table, any expressions in constant position are
+checked to make sure they are actually constant. This includes `const`
+declarations, the right-hand-side of `enum` values, and the inside of
+`export` clauses.
+
 Then the IR is compiled into an internal representation of GDScript.
 This is basically GDScript source code, but represented as an abstract
 syntax tree manipulable within the GDLisp engine. This compilation