[atomics.order] Use "recommended practice" (#6380)

Author: Eisenwave

source/threads.tex

@@ -2646,8 +2646,10 @@
 the read-modify-write operation.
 
 \pnum
-Implementations should make atomic stores visible to atomic loads within a reasonable
-amount of time.
+\recommended
+The implementation should make atomic stores visible to atomic loads,
+and atomic loads should observe atomic stores,
+within a reasonable amount of time.
 
 \indexlibraryglobal{kill_dependency}%
 \begin{itemdecl}