doc|config: Nordic RF5 fix references to SOC_NRF5

There was a reference to SOC_FAMILY_NRF5 which was not use by anyone
else and it seemes like it was a typo, where the right reference was
to symbol SOC_NRF5. However, the right fix is to move both sites to
use SOC_FAMILY_NRF5 *and* fix the SOC_SERIES to be only nrf52,
otherwise it causes path duplication and the build fails.

This was also causing documentation warnings that are thus killed.

Change-Id: I92e74a6158f02df43e6e857df8f1e67bcfdd9551
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>