Rename the parameter `ptr` of `notes_init` to something meaningful. Since
it points to the start of the notes segment rename it to `segment_start`.
In addition, improve the documentation for the function.
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>