QString: fix documentation of NUL-termination of unicode(), data(), constData()

(or lack thereof).

* `QString::utf16()` reallocates if `*this` has been constructed using
  `fromRawData()`.

* `QString::data()` ensures a writable string, so will detach from raw
  data.

* `QString::unicode()`, `constData()`, and `data() const` do not. They
  just return `QStringData::data()`, which may point to raw,
  non-NUL-terminated data.

These functions can therefore not possibly have the same behavior, but
were documented the same.

Fix.

Also drop the discussion of operator[](size()), as that, too, was not
correct, and anyone who indexes with op[] beyond size() - 1 should not
rely on proper behavior.

[ChangeLog][QtCore][QString] QString::unicode(), constData() and
`data() const` do not return a NUL-terminated string. This was true
before, but the documentation claimed the opposite.

Change-Id: I1437f57cd02bdf80264e8559608b46aa749c23a8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/doc/snippets/qstring/main.cpp

@@ -733,7 +733,6 @@ void Widget::sizeFunction()
     int n = str.size();         // n == 5
     str.data()[0];              // returns 'W'
     str.data()[4];              // returns 'd'
-    str.data()[5];              // returns '\0'
     //! [58]
 }
 

src/corelib/tools/qstring.cpp

@@ -5138,14 +5138,9 @@ QString& QString::fill(QChar ch, int size)
 
     Returns the number of characters in this string.
 
-    The last character in the string is at position size() - 1. In
-    addition, QString ensures that the character at position size()
-    is always '\\0', so that you can use the return value of data()
-    and constData() as arguments to functions that expect
-    '\\0'-terminated strings.
+    The last character in the string is at position size() - 1.
 
     Example:
-
     \snippet qstring/main.cpp 58
 
     \sa isEmpty(), resize()
@@ -5674,10 +5669,13 @@ int QString::localeAwareCompare_helper(const QChar *data1, int length1,
 /*!
     \fn const QChar *QString::unicode() const
 
-    Returns a '\\0'-terminated Unicode representation of the string.
+    Returns a Unicode representation of the string.
     The result remains valid until the string is modified.
 
-    \sa utf16()
+    \note The returned string may not be '\\0'-terminated.
+    Use size() to determine the length of the array.
+
+    \sa utf16(), fromRawData()
 */
 
 /*!
@@ -8077,7 +8075,10 @@ bool QString::isRightToLeft() const
 
     Returns a pointer to the data stored in the QString. The pointer
     can be used to access and modify the characters that compose the
-    string. For convenience, the data is '\\0'-terminated.
+    string.
+
+    Unlike constData() and unicode(), the returned data is always
+    '\\0'-terminated.
 
     Example:
 
@@ -8093,18 +8094,25 @@ bool QString::isRightToLeft() const
 /*! \fn const QChar *QString::data() const
 
     \overload
+
+    \note The returned string may not be '\\0'-terminated.
+    Use size() to determine the length of the array.
+
+    \sa fromRawData()
 */
 
 /*! \fn const QChar *QString::constData() const
 
     Returns a pointer to the data stored in the QString. The pointer
-    can be used to access the characters that compose the string. For
-    convenience, the data is '\\0'-terminated.
+    can be used to access the characters that compose the string.
 
     Note that the pointer remains valid only as long as the string is
     not modified.
 
-    \sa data(), operator[]()
+    \note The returned string may not be '\\0'-terminated.
+    Use size() to determine the length of the array.
+
+    \sa data(), operator[](), fromRawData()
 */
 
 /*! \fn void QString::push_front(const QString &other)