qemu: document GDB user mode, fix some stuff

README.adoc

@@ -7699,6 +7699,26 @@ It is already usable with:
 
 TODO: factor <<qemu-user-mode>> with this:
 
+===== QEMU user mode GDB
+
+It's nice when the obvious works, right?
+
+....
+./run --debug-guest --user ./x86_64.out
+....
+
+and on another shell:
+
+....
+./run-gdb --user ./x86_64.out main
+....
+
+or to stop at the very first instruction of a freestanding program:
+
+....
+./run-gdb --no-continue --user ./x86_64.out
+....
+
 ==== gem5 syscall emulation mode
 
 Analogous to <<qemu-user-mode>>, but less usable:
@@ -7752,7 +7772,7 @@ Then I was told that it is has never been tested outside of x86_64:
 ===== gem5 syscall emulation mode CLI options
 
 ....
-./run --gem5 --user ./aarch64.out -- --options 'op1 "op 2" op3'
+./run --gem5 --user ./x86_64.out -- --options 'op1 "op 2" op3'
 ....
 
 ==== User mode vs full system benchmark