Add 'foreign linker API' in 'jdk.incubator.foreign' module desc/summary

The foreign linker API (incubator) has been introduced in EA16 build 26.
The `jdk.incubator.foreign` module description and summary should be updated to reflect this as only "foreign memory access API" is mentioned.

See
https://download.java.net/java/early_access/jdk16/docs/api/
https://download.java.net/java/early_access/jdk16/docs/api/jdk.incubator.foreign/module-summary.html