Bug 574603 - [null][external] offer more IDE options for specifying
external annotation locations

user documentation

Change-Id: I1160c0160460068b462bb5c35eec3f1a0b7b6f01
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.common/+/183349
Tested-by: Stephan Herrmann <stephan.herrmann@berlin.de>
Reviewed-by: Stephan Herrmann <stephan.herrmann@berlin.de>