tracing::trace_state.hh: Add descriptions for main methods and functions
tracing::trace_state.hh: Add descriptions for main methods and functions

Add a proper description to a tracing::trace() that clarifies
that the tracing message string and the positional parameters
are going to be copied if tracing state is initialized.

Add a description for trace_state::begin() methods and for a
tracing::begin() helper function.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>

avikivity/scylla,kjniemi/scylla,kjniemi/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla