Manual.rst: Add info for 'letter-no-break'

Documentation/Manual.rst

@@ -7602,6 +7602,8 @@ QFQ CSS Classes
 * `qfq-100` - assigned to different tags, makes an element 'width: 100%'.
 * `qfq-left`- assigned to different tags, Text align left.
 * `qfq-sticky` - assigned to `<thead>`, makes the header sticky.
+* `letter-no-break` - assigned to a  `div` will protect a paragraph (CSS: page-break-before: avoid;) not to break and
+  page border, when converted to PDF via wkhtml. Take care that qfq-letter.css is included in TypoScript Setup.
 
 Bootstrap
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