[doc] Disambiguate doc around ECC detection on the data interface

The Ibex documentation currently specifies:
https://ibex-core.readthedocs.io/en/latest/03_reference/security.html#bus-integrity-checking

When the SecureIbex parameter is set, incoming data will be checked against the supplied checkbits.

https://ibex-core.readthedocs.io/en/latest/03_reference/load_store_unit.html#bus-integrity-checking

The core can optionally generate and verify check bits sent alongside the data for memory accesses.

The former statement suggests that all received data is checked (also when no load/store is going on) which is what is really happening. While the latter suggests that only response data of actual load/store instructions is checked. We should correct the second statement to disambiguate the documentation here.

This got flagged by Yin Mengyuan and @KatCe .