doc: update to package.exports docs

doc/api/esm.md

@@ -191,6 +191,12 @@ also be used within `"exports"` to define different package entry points per
 environment, including whether the package is referenced via `require` or via
 `import`.
 
+**Warning**: Introducing the `"exports"` field prevents consumers of a package
+from using any entry points that are not defined, including the `package.json`.
+This will likely be a breaking change. To make the introduction of `"exports"`
+non-breaking, either ensure that every previously supported entry point is
+exported or include an export of the root of the package `"./": "./"`.
+
 If both `"exports"` and `"main"` are defined, the `"exports"` field takes
 precedence over `"main"`.