[wasm] Document what StringLiteral is

libyul/backends/wasm/BinaryTransform.cpp

@@ -345,8 +345,9 @@ bytes BinaryTransform::operator()(Literal const& _literal)
 
 bytes BinaryTransform::operator()(StringLiteral const&)
 {
-	// TODO is this used?
-	yulAssert(false, "String literals not yet implemented");
+	// StringLiteral is a special AST element used for certain builtins.
+	// It is not mapped to actual WebAssembly, and should be processed in visit(BuiltinCall).
+	yulAssert(false, "");
 }
 
 bytes BinaryTransform::operator()(LocalVariable const& _variable)

libyul/backends/wasm/TextTransform.cpp

@@ -101,6 +101,8 @@ string TextTransform::operator()(wasm::Literal const& _literal)
 
 string TextTransform::operator()(wasm::StringLiteral const& _literal)
 {
+	// StringLiteral is a special AST element used for certain builtins.
+	// The output of this will not be valid WebAssembly.
 	string quoted = boost::replace_all_copy(_literal.value, "\\", "\\\\");
 	boost::replace_all(quoted, "\"", "\\\"");
 	return "\"" + quoted + "\"";

libyul/backends/wasm/WasmAST.h

@@ -63,6 +63,7 @@ using Expression = std::variant<
 >;
 
 struct Literal { std::variant<uint32_t, uint64_t> value; };
+// This is a special AST element used for certain builtins. It is not mapped to actual WebAssembly.
 struct StringLiteral { std::string value; };
 struct LocalVariable { std::string name; };
 struct GlobalVariable { std::string name; };