scripts/gdb: update documentation for lx_per_cpu

scripts/gdb: update documentation for lx_per_cpu

Commit db08c53fdd542bb7f83b ("scripts/gdb: fix parameter handling in
$lx_per_cpu") changed the parameter handling of lx_per_cpu to use GdbValue
instead of parsing the variable name.  Update the documentation to reflect
the new lx_per_cpu usage.  Update the hrtimer_bases example to use rb_tree
instead of the timerqueue_head.next pointer removed in commit
511885d7061eda3eb1fa ("lib/timerqueue: Rely on rbtree semantics for next
timer").

Link: https://lkml.kernel.org/r/20250503123234.2407184-3-illia@yshyn.com
Signed-off-by: Illia Ostapyshyn <illia@yshyn.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Dongliang Mu <dzm91@hust.edu.cn>
Cc: Florian Rommel <mail@florommel.de>
Cc: Hu Haowen <2023002089@link.tyut.edu.cn>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Yanteng Si <si.yanteng@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>