Document monotonic mutability of the AST (#1079)

Also document an important precondition on `InterpExp` and `InterpPattern`

executable_semantics/ast/README.md

@@ -7,6 +7,16 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 The code in this directory defines the AST that represents Carbon code in the
 rest of `executable-semantics`.
 
+The AST is not quite immutable, because some node properties are set during some
+phase of static analysis, rather than during parsing. However, AST mutations are
+_monotonic_: once set, a node property cannot be changed. Furthermore, if a
+property is set after parsing, its documentation specifies what phase is
+responsible for setting it. Certain properties have `has_foo()` members for
+querying whether they are set, but those are for internal use within the phase
+that sets them. As a result, you can think of the AST as if it were immutable,
+but with certain parts that you can't yet observe, depending on what phase of
+compilation you're in.
+
 All node types in the AST are derived from [`AstNode`](ast_node.h), and use
 [LLVM-style RTTI](https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html) to support
 safe down-casting and similar operations. Each abstract class `Foo` in the