P3878R1 Standard library hardening should not use the 'observe' semantic

Fixes NB RU 016, FR 001-014, FR 010-113, US 3-015, US 61-112 (C++26 CD).

Author: jwakely

source/lib-intro.tex

@@ -390,12 +390,9 @@
 \item
 When invoking the function in a hardened implementation,
 prior to any other observable side effects of the function,
-one or more contract assertions
+contract assertions
 whose predicates are as described in the hardened precondition
-are evaluated with a checking semantic\iref{basic.contract.eval}.
-If any of these assertions is evaluated with a non-terminating semantic
-and the contract-violation handler returns,
-the program has undefined behavior.
+are evaluated with a terminating semantic\iref{basic.contract.eval}.
 \item
 When invoking the function in a non-hardened implementation,
 if any hardened precondition is violated,