Bug 572890: Add @launcher.dir to documentation

Contributed by STMicroelectronics

Change-Id: I52e62f3979c8cb8f6d13e66e67eeaf2761dfa7d1
Signed-off-by: Torbjörn Svensson <torbjorn.svensson@st.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.common/+/181146
Reviewed-by: Jonah Graham <jonah@kichwacoders.com>
Reviewed-by: Thomas Watson <tjwatson@us.ibm.com>
Tested-by: Thomas Watson <tjwatson@us.ibm.com>