core_ast: remove letrec comment

doc/env.md

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+This document holds my design notes for lexical and global environments
+for this compiler. I have not yet named the language.
+
+# Environments
+
+An environment is an integral part of the runtime of the language.
+There is a global environment that holds the values of all global
+symbols.
+
+Lexical environments generally don't exist in practice, instead we use
+flat closures. When a closure is created at runtime, all free variables
+it uses are packaged as part of the function object, then the function
+body uses a GetFree instruction to get those free variables by an index.
+
+Free variables are propagated from inner closures outwards. This is necessary,
+as this also handles multiple-argument functions gracefully.
+
+
+```scheme
+(let ((a 10))
+  (print (+ a 5)))
+```
+
+
+## Global Definitions
+
+Any symbol defined through a top-level `define` form is made globally available
+after the definition form.
+
+This is the most common use for define.
+
+Generally any symbol appearing in the body of a function, will only be compiled
+to access that symbol. The symbol is only accessed once the function is called.
+Thus, you can create mutually recursive functions at the top level with no issue.
+
+## Local Definitions
+
+It is valid to use `define` forms in body sections. Informally, a body section
+is the body of most built-in forms, including `lambda`, `let`, and `letrec`.
+
+