bluetooth: conn: Fix forced pairing request handling

BT_SECURITY_FORCE_PAIR option gets overridden when CONFIG_BT_SMP_SC_ONLY
or CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY is defined. Cache the bit value
before overrides.
Add explicit forced pair handling to existing security level check.
Functionality did not change as this was done implicitly due to integer
comparison for enums.
Add extra clarification to the method doc.
Signed-off-by: Rait Rääk <raitraak@gmail.com>