[std] Replace 'must'.

Partially addresses ISO/CS 017 (C++20 DIS)

Author: jensmaurer

source/classes.tex

@@ -3603,7 +3603,7 @@
 different from the polymorphic behavior of a most derived object of the
 same type. A base class subobject can be of zero size;
 however, two subobjects that have the same class type and that belong to
-the same most derived object must not be allocated at the same
+the same most derived object cannot be allocated at the same
 address\iref{intro.object}.
 \end{note}
 
@@ -6621,7 +6621,7 @@
 overload resolution is performed again,
 considering the expression or operand as an lvalue.
 \begin{note}
-This two-stage overload resolution must be performed regardless
+This two-stage overload resolution is performed regardless
 of whether copy elision will occur. It determines the constructor
 or the \tcode{return_value} overload to be called if
 elision is not performed, and the selected constructor

source/lex.tex

@@ -57,8 +57,8 @@
 \pnum
 \indextext{translation!phases|(}%
 The precedence among the syntax rules of translation is specified by the
-following phases.\footnote{Implementations must behave as if these separate phases
-occur, although in practice different phases might be folded together.}
+following phases.\footnote{Implementations behave as if these separate phases
+occur, although in practice different phases can be folded together.}
 
 \begin{enumerate}
 \item