Update documentation to clarify how to set Global Capabilities

The documentation does not clearly state that the Global Capabilities
are set via the access rights of the root project (All-Projects).

Also the section in which the Global Capabilities are documented is
named "System Capabilities" which makes it harder to find by searching.

Change the name of the section, and add a clarification paragraph.

Change-Id: I46eaf1e81dfc4962f482571fe25b6a6a10e17b45
2 files changed