doc: add gdb commands to log output in a file (user guide)

doc/de/weechat_user.de.txt

@@ -246,6 +246,9 @@ Nun startet man 'gdb' und führt den Befehl `bt full` innerhalb gdb aus um die F
 auszugeben. Eine Fehlermeldung von gdb sieht z.B. wie folgt aus:
 
 ----------------------------------------
+(gdb) set logging file /tmp/crash.txt
+(gdb) set logging on
+Copying output to /tmp/crash.txt.
 (gdb) bt full
 #0  0x00007f9dfb04a465 in raise () from /lib/libc.so.6
 #1  0x00007f9dfb04b8e6 in abort () from /lib/libc.so.6

doc/en/weechat_user.en.txt

@@ -247,6 +247,9 @@ Then under gdb, use command `bt full` to display backtrace.
 You will see something like that:
 
 ----------------------------------------
+(gdb) set logging file /tmp/crash.txt
+(gdb) set logging on
+Copying output to /tmp/crash.txt.
 (gdb) bt full
 #0  0x00007f9dfb04a465 in raise () from /lib/libc.so.6
 #1  0x00007f9dfb04b8e6 in abort () from /lib/libc.so.6

doc/fr/weechat_user.fr.txt

@@ -254,6 +254,9 @@ Puis sous gdb, utilisez la commande `bt full` pour afficher la trace.
 Vous verrez quelque chose comme ceci :
 
 ----------------------------------------
+(gdb) set logging file /tmp/crash.txt
+(gdb) set logging on
+Copying output to /tmp/crash.txt.
 (gdb) bt full
 #0  0x00007f9dfb04a465 in raise () from /lib/libc.so.6
 #1  0x00007f9dfb04b8e6 in abort () from /lib/libc.so.6

doc/it/weechat_user.it.txt

@@ -255,6 +255,9 @@ All'interno di gdb, usare il comando `bt full` per visualizzare
 il backtrace. Verrà mostrato qualcosa di simile a questo:
 
 ----------------------------------------
+(gdb) set logging file /tmp/crash.txt
+(gdb) set logging on
+Copying output to /tmp/crash.txt.
 (gdb) bt full
 #0  0x00007f9dfb04a465 in raise () from /lib/libc.so.6
 #1  0x00007f9dfb04b8e6 in abort () from /lib/libc.so.6