[lib] Replace "shall not X" with "does not X" when it describes library behavior.

In these places we aren't saying "it's UB if X happens"; we're literally
specifying the behavior of a library function as "X does not happen,
we promise."

Jonathan Wakely points out that there is still room for the user to cause UB
by specializing `pair`, `duration`, `function`, etc. such that their specializations
do X. In that case, the UB happens due to [namespace.std] p2, which requires
that "the [program-defined] specialization meets the standard library requirements
for the original template."

Author: Quuxplusone

source/time.tex

@@ -1254,7 +1254,7 @@
 If \tcode{Period::num} is not positive, the program is ill-formed.
 
 \pnum
-Members of \tcode{duration} shall not throw exceptions other than
+Members of \tcode{duration} do not throw exceptions other than
 those thrown by the indicated operations on their representations.
 
 \pnum

source/utilities.tex

@@ -512,13 +512,13 @@
 \end{codeblock}
 
 \pnum
-Constructors and member functions of \tcode{pair} shall not throw exceptions unless one of
+Constructors and member functions of \tcode{pair} do not throw exceptions unless one of
 the element-wise operations specified to be called for that operation
 throws an exception.
 
 \pnum
-The defaulted move and copy constructor, respectively, of \tcode{pair} shall
-be a constexpr function if and only if all required element-wise
+The defaulted move and copy constructor, respectively, of \tcode{pair}
+is a constexpr function if and only if all required element-wise
 initializations for copy and move, respectively, would satisfy the
 requirements for a constexpr function.
 
@@ -1156,10 +1156,10 @@
 
 \pnum
 The defaulted move and copy constructor, respectively, of
-\tcode{tuple} shall be a constexpr function if and only if all
+\tcode{tuple} is a constexpr function if and only if all
 required element-wise initializations for copy and move, respectively,
 would satisfy the requirements for a constexpr function. The
-defaulted move and copy constructor of \tcode{tuple<>} shall be
+defaulted move and copy constructor of \tcode{tuple<>} are
 constexpr functions.
 
 \pnum
@@ -15511,7 +15511,7 @@
 
 \pnum
 \throws
-Shall not throw exceptions if \tcode{f}'s target is
+Nothing if \tcode{f}'s target is
 a specialization of \tcode{reference_wrapper} or
 a function pointer. Otherwise, may throw \tcode{bad_alloc}
 or any exception thrown by the copy constructor of the stored callable object.
@@ -15582,10 +15582,10 @@
 
 \pnum
 \throws
-Shall not throw exceptions when \tcode{f} is a function pointer
-or a \tcode{reference_wrapper<T>} for some \tcode{T}. Otherwise,
-may throw \tcode{bad_alloc} or any exception thrown by \tcode{F}'s copy
-or move constructor.
+Nothing if \tcode{f} is
+a specialization of \tcode{reference_wrapper} or
+a function pointer. Otherwise, may throw \tcode{bad_alloc}
+or any exception thrown by \tcode{F}'s copy or move constructor.
 \end{itemdescr}