Document that every ID in a REST URL must be URL encoded

Document this once for the whole REST API instead of writing this for
every REST endpoint again and again.

Change-Id: Ic919d202316a1bef5042732a415bd5b43d75f40f
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/rest-api.txt

diff --git a/Documentation/rest-api.txt b/Documentation/rest-api.txt
index 138c7c1..20108cd 100644
--- a/Documentation/rest-api.txt
+++ b/Documentation/rest-api.txt
@@ -61,6 +61,12 @@
 `Accept-Encoding` request header is set to `gzip`. This may
 save on network transfer time for larger responses.
 
+[[encoding]]
+Encoding
+~~~~~~~~
+All IDs that appear in the URL of a REST call (e.g. project name, group name)
+must be URL encoded.
+
 Endpoints
 ---------
 link:rest-api-accounts.html[/accounts/]::