Introduce "--shm-path-ust" to clarify use vs documentation
Considering that the "--shm-path" argument is documented as applying to all domains, but that currently the kernel domain does not support this argument, it leads us to a situation where users may budget their NVRAM for the space needed only for UST buffers, and eventually upgrade to a new LTTng version which would implement kernel support for shm-path. They would then face issues given the lack of available space.
I would recommend to add a new "--shm-path-ust" argument as an alias to "--shm-path", but document that it only applies to the UST domain. This way, users wishing to budget their NVRAM space only for UST tracing, but still perform kernel tracing in the same session, will be able to do so.