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

Documentation/rest-api.txt

diff --git a/Documentation/rest-api.txt b/Documentation/rest-api.txt
index c2172d7..303fc4b 100644
--- a/Documentation/rest-api.txt
+++ b/Documentation/rest-api.txt
@@ -112,7 +112,7 @@
 calling user has no sufficient permissions.
 
 E.g. some REST endpoints require that the calling user has certain
-link:access-control.html#system_capabilities[global capabilities]
+link:access-control.html#global_capabilities[global capabilities]
 assigned.
 
 `403 Forbidden` is also used if `self` is used as account ID and the