scripts: west_commands: sdk: clarify toolchain selection help

Users were confused about which toolchain names to pass to
`west sdk install -t`, since the documentation did not list them.
Enhance the help text to suggest installing all toolchains if
unsure and point to the SDK release notes where the available
toolchains are listed.

ref: https://github.com/zephyrproject-rtos/zephyr/discussions/94572



Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>