ftrace: Add comments to ftrace_hash_move() and friends

ftrace: Add comments to ftrace_hash_move() and friends

Describe what ftrace_hash_move() does and add some more comments to some
other functions to make it easier to understand.

Link: https://lore.kernel.org/linux-trace-kernel/20240605180409.179520305@goodmis.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>