The paths from which GPRinstall is supposed to copy the documentation seems incorrect. Looks like a missing indirection.
--- a/lib/gnat/aunit.gpr
+++ b/lib/gnat/aunit.gpr
@@ -50,13 +50,13 @@
package Install is
for Artifacts ("share/doc/aunit/pdf")
- use ("../../doc/pdf/**");
+ use ("../../doc/build/aunit_cb/pdf/**");
for Artifacts ("share/doc/aunit/txt")
- use ("../../doc/txt/**");
+ use ("../../doc/build/aunit_cb/txt/**");
for Artifacts ("share/doc/aunit/info")
- use ("../../doc/info/**");
+ use ("../../doc/build/aunit_cb/info/**");
for Artifacts ("share/doc/aunit/html")
- use ("../../doc/html/**");
+ use ("../../doc/build/aunit_cb/html/**");
for Artifacts ("share/gps/plug-ins") use ("../../support/aunit.xml");
for Artifacts ("share/examples/aunit") use ("../../examples/*");