[docs] Separate two paragraphs in max_size() docs

Formatting indicates that this was how it should be rendered, but qdoc
won't paragraph-break unless there's an empty line.

Add it.

Amends 7ce6920aacfcba485cd8017e01c6aeb324292e75.

Change-Id: Idfd3cbc9f075b5f206c2042dc7b22508735dcb38
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 02b436f2c5d7368827794e7ec1224419dca39e7f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qbytearray.cpp

@@ -1424,6 +1424,7 @@ QByteArray &QByteArray::operator=(const char *str)
     \since 6.8
 
     This function is provided for STL compatibility.
+
     It returns the maximum number of elements that the byte array can
     theoretically hold. In practice, the number can be much smaller,
     limited by the amount of memory available to the system.

src/corelib/text/qstring.cpp

@@ -6418,6 +6418,7 @@ QString& QString::fill(QChar ch, qsizetype size)
     \since 6.8
 
     This function is provided for STL compatibility.
+
     It returns the maximum number of elements that the string can
     theoretically hold. In practice, the number can be much smaller,
     limited by the amount of memory available to the system.

src/corelib/tools/qlist.qdoc

@@ -1336,6 +1336,7 @@
     \since 6.8
 
     This function is provided for STL compatibility.
+
     It returns the maximum number of elements that the list can
     theoretically hold. In practice, the number can be much smaller,
     limited by the amount of memory available to the system.

src/corelib/tools/qvarlengtharray.qdoc

@@ -144,6 +144,7 @@
     \since 6.8
 
     This function is provided for STL compatibility.
+
     It returns the maximum number of elements that the array can
     theoretically hold. In practice, the number can be much smaller,
     limited by the amount of memory available to the system.