doc: sandbox: Add a note of disabling LTO when using gdb

Image created by LTO is not friendly to debugger, let's document this.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

Mention CONFIG_CC_OPTIMIZE_FOR_DEBUG and LLDB.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/arch/sandbox.rst b/doc/arch/sandbox.rst
index 246ab5b..bfc9cc6 100644
--- a/doc/arch/sandbox.rst
+++ b/doc/arch/sandbox.rst
@@ -477,6 +477,9 @@
 
 This approach can be used on normal boards as well as sandbox.
 
+For debugging with GDB or LLDB, it is preferable to reduce the compiler
+optimization level (CONFIG_CC_OPTIMIZE_FOR_DEBUG=y) and to disable Link Time
+Optimization (CONFIG_LTO=n).
 
 SDL_CONFIG
 ----------