[basic.string] State class invariants about [data(), data() + size()] in the introduction. (#1879)

Author: tkoeppe

source/strings.tex

@@ -853,7 +853,10 @@
 
 \pnum
 In all cases,
-\tcode{size() <= capacity()}.
+\crange{data()}{data() + size()} is a valid range,
+\tcode{data() + size()} points at an object with value \tcode{charT()}
+(a ``null terminator''\indextext{string!null terminator}),
+and \tcode{size() <= capacity()} is \tcode{true}.
 
 \pnum
 The functions described in this Clause can report two
@@ -1202,8 +1205,7 @@
 
 \pnum
 \postconditions
-\tcode{data()} is a non-null pointer that is copyable and can have \tcode{0} added
-to it, \tcode{size()} is \tcode{0}, and \tcode{capacity()} is an unspecified value.
+\tcode{size()} is \tcode{0} and \tcode{capacity()} is an unspecified value.
 \end{itemdescr}
 
 \indexlibrary{\idxcode{basic_string}!constructor}%