Revert "doc: posix: mark posix signals supported with undefined behaviour"

This reverts commit b10f1ca3.

PR #73978 introduced a regression.
Unfortunately this PR cannot be reverted without reverting also
Let's revert both PRs to stabilize main again towards the 3.7 release.

For more details on the issue see
https://github.com/zephyrproject-rtos/zephyr/issues/75205



Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>