chore: documentation

Author: TomAFrench

compiler/noirc_frontend/src/hir/type_check/expr.rs

@@ -154,6 +154,8 @@ impl<'interner> TypeChecker<'interner> {
                 // Need to setup these flags here as `self` is borrowed mutably to type check the rest of the call expression
                 // These flags are later used to type check calls to unconstrained functions from constrained functions
                 let Some(current_func) = self.current_function else {
+                        // If we don't have a value for `current_function` then we are initializing a global.
+                        // We disallow these from being assigned from a function call.
                         self.errors.push(TypeCheckError::GlobalFunctionCall {
                             span: self.interner.expr_span(expr_id),
                         });