Offset before square bracket in x86 intel asm on GCC

From all the docs I've found, there is no mention of syntax like offset[var+offset2] in Intel x86 syntax but GCC with the following flags

gcc -S hello.c -o - -masm=intel

for this program

#include<stdio.h>int main(){    char c = 'h';    putchar(c);    return 0;}

produces

    .file   "hello.c"    .intel_syntax noprefix    .text    .globl  main    .type   main, @functionmain:.LFB0:    .cfi_startproc    push    rbp    .cfi_def_cfa_offset 16    .cfi_offset 6, -16    mov rbp, rsp    .cfi_def_cfa_register 6    sub rsp, 16    mov BYTE PTR -1[rbp], 104    movsx   eax, BYTE PTR -1[rbp]    mov edi, eax    call    putchar@PLT    mov eax, 0    leave    .cfi_def_cfa 7, 8    ret    .cfi_endproc.LFE0:    .size   main, .-main    .ident  "GCC: (Arch Linux 9.3.0-1) 9.3.0"    .section    .note.GNU-stack,"",@progbits

I'd like to highlight the line mov BYTE PTR -1[rbp], 104 where offset -1 appears outside the square brackets. TBH, I'm just guessing that it is an offset, can anyone direct me to a proper documentation highlighting this ?

Here is a similar question: Squared Brackets in x86 asm from IDA where a comment does mention that it is an offset but I'd really like a proper documentation reference.