Update description about GET /projects endpoint

It returns all projects visible for the authenticated user, not just
the project they are member of.

doc/api/projects.md

@@ -20,7 +20,7 @@ Constants for project visibility levels are next:
 ## List projects
-Get a list of projects for which the authenticated user is a member.
+Get a list of visible projects for authenticated user. When being accessed without authentication, all public projects are returned.
 ```
 GET /projects