Now that unified-shared memory works (with some devices), mark it as 'Y' 
and link to the device-specific chapter. While there is always room for 
improvement (like having opt-in partial support for managed-memory 
semi-USM devices), it works sufficienty for a 'Y'.

Additionally, I saw that 5.2 now extended what is permitted inside 
'declare mapper'. Instead of listening the permitted clauses as in 5.1, 
it now refers to the 'map' clause such that 'delete'/'release', 
'present' and in particular 'iterator' and 'mapper' itself are permitted 
inside a declare-mapper 'map' clause. - Thus, I added it as to-do item 
to the 5.2 status.

Comments?

Tobias

PS: As this is also about USM, the declare-target USM issue I mentioned 
in several patch emails is now filed as https://gcc.gnu.org/PR115279