The following test code shows that Q flag nor any other flags are changed by uqadd8.
uint32_t a = 0x00050000;
uint32_t b = 0x00f00000;
uint32_t apsr = 0xffffffff;
uint32_t res = 0;
asm volatile (R"asm(
mrs r5, APSR
bic r5, r5, #0xf8000000
msr APSR_nzcvq, r5
uqadd8 %[res], %[a], %[b]
mrs %[apsr], APSR
)asm"
: [apsr] "+r" (apsr), [res] "+r" (res)
: [a] "r" (a), [b] "r" (b)
: "r5"
);
APRS remain 0 for both a,b = 0x00050000,0x00f00000 and a,b = 0x00550000,0x00f00000. The result saturates as expected, but there is no detectable change in flags.
So ARM documentation appears to be correct in this case.
The test was performed on Cortex-M4F (SAM E54).