openrisc: Cleanup copy_thread_tls docs and comments

Previously copy_thread_tls was copy_thread and before that something
else.  Remove the documentation about the regs parameter that didn't
exist in either version.

Next, fix comment wrapping and details about how TLS pointer gets to the
copy_thread_tls function.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>