kva_alloc: document linear mapping overlap with boot.S

kva_alloc: document linear mapping overlap with boot.S

Add documentation note explaining that the KVA allocator region
(0xC0500000..0xC0800000) falls within the initial 16MB linear mapping
set up by boot.S (0xC0000000..0xC0100000). This means V2P() works by
coincidence for addresses in this range, but code should use
vmm_virt_to_phys() instead to be portable and future-proof.

This documents the architectural fragility identified during H6
regression analysis.

Test: make test-battery PASS (157/157), make analyzer PASS