rslib: Update documentation

The decoder returns the number of corrected symbols, not bits.
The caller provided syndrome must be in index form.

Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-7-ferdinand.blomqvist@gmail.com