Update QString documentation relevant to prepend optimization

[ChangeLog][QtCore][QString] QString is a prepend optimized
container similar to QList.

Task-number: QTBUG-84320
Change-Id: Id035aac859a6f404e389f2eeaf20f9eee5bff20c
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/corelib/text/qstring.cpp

@@ -1981,6 +1981,11 @@ inline char qToLower(char ch)
     Returns an \l{STL-style iterators}{STL-style iterator} pointing to the first character in
     the string.
 
+//! [iterator-invalidation-func-desc]
+    \warning The returned iterator is invalidated on detachment or when the
+    QString is modified.
+//! [iterator-invalidation-func-desc]
+
     \sa constBegin(), end()
 */
 
@@ -1995,6 +2000,8 @@ inline char qToLower(char ch)
     Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first character
     in the string.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa begin(), cend()
 */
 
@@ -2003,6 +2010,8 @@ inline char qToLower(char ch)
     Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first character
     in the string.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa begin(), constEnd()
 */
 
@@ -2011,6 +2020,8 @@ inline char qToLower(char ch)
     Returns an \l{STL-style iterators}{STL-style iterator} pointing to the imaginary character
     after the last character in the string.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa begin(), constEnd()
 */
 
@@ -2025,6 +2036,8 @@ inline char qToLower(char ch)
     Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary
     character after the last character in the list.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa cbegin(), end()
 */
 
@@ -2033,6 +2046,8 @@ inline char qToLower(char ch)
     Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary
     character after the last character in the list.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa constBegin(), end()
 */
 
@@ -2042,6 +2057,8 @@ inline char qToLower(char ch)
     Returns a \l{STL-style iterators}{STL-style} reverse iterator pointing to the first
     character in the string, in reverse order.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa begin(), crbegin(), rend()
 */
 
@@ -2056,6 +2073,8 @@ inline char qToLower(char ch)
     Returns a const \l{STL-style iterators}{STL-style} reverse iterator pointing to the first
     character in the string, in reverse order.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa begin(), rbegin(), rend()
 */
 
@@ -2065,6 +2084,8 @@ inline char qToLower(char ch)
     Returns a \l{STL-style iterators}{STL-style} reverse iterator pointing to one past
     the last character in the string, in reverse order.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa end(), crend(), rbegin()
 */
 
@@ -2079,6 +2100,8 @@ inline char qToLower(char ch)
     Returns a const \l{STL-style iterators}{STL-style} reverse iterator pointing to one
     past the last character in the string, in reverse order.
 
+    \include qstring.cpp iterator-invalidation-func-desc
+
     \sa end(), rend(), rbegin()
 */
 
@@ -2422,6 +2445,10 @@ void QString::resize(qsizetype size, QChar fillChar)
     \note a statically allocated string will report a capacity of 0,
     even if it's not empty.
 
+    \note The free space position in the allocated memory block is undefined. In
+    other words, one should not assume that the free memory is always located
+    after the initialized elements.
+
     \sa reserve(), squeeze()
 */