Document a bug when processes hang on uvn_fp2. FWIW, hung process exhibit
this stack backtrace:
sleepq_block
mtsleep
uvn_findpage
uvn_findpages
genfs_getpages
puffs_vnop_getpages
puffs_vnop_checkop
VOP_GETPAGES
uvn_get
uvm_fault_internal
trap
copyin
uiomove
ubc_uiomove
puffs_vnop_write
puffs_vnop_checkop
VOP_WRITE
vn_write
dofilewrite
sys_write
syscall

(manu)