Update documentation about em/en dashes and bitmap fonts.

lib/RELEASE-NOTES

@@ -13,11 +13,13 @@
   be safely dissolved, as it will be automatically inserted at export time
   if needed, as usual.
 
-* LyX now outputs en- and em-dashes as -- and --- ligatures when exporting to
-  latex using TeX fonts, as done in version 2.1 and earlier. In version 2.2
-  they were instead output as the macros \textendash and \textemdash, causing
-  changed output with old documents and bugs. The 2.2 behavior can be restored
-  by don't allowing using dash ligatures in Document->Settings->Fonts.
+* The new setting
+  "Document->Settings->Fonts->Output em- and en-dash as ligatures" forces
+  output of en- and em-dashes as -- and --- when exporting to LaTeX.
+  It is is "true" by default but "false" when opening documents edited
+  with LyX 2.2.
+  See chapter 3.9.1.1 "Dashes and line breaks" of the User Guide and
+  "Caveats when upgrading from earlier versions to 2.3.x" below.
 
 * The following UI translations were dropped, because the lack of translation
   maintenance:  Russian, Danish, Greek, Serbian, Galician, Catalan, Romanian,
@@ -202,6 +204,16 @@
   the external_templates file, you will have to move the modifications to
   the respective *.xtemplate file manually.
 
+* If you used literal em- and en-dashes in pre-2.2 documents,
+  you must manually unselect
+  "Document->Settings->Fonts->Output em- and en-dash as ligatures"
+  to ensure unchanged behaviour.
+
+* ZWSP characters (u200b) following literal em- and en-dashes are deleted by
+  lyx2lyx when converting to 2.3 format. If you used them as optional line
+  breaks after dashes, convert them to space insets before opening your
+  document with LyX 2.3 or the optional line breaks will be lost!
+
 * If using TeX fonts and en- and em-dashes are output as font ligatures,
   when exporting documents containing en- and em-dashes to the format of
   LyX 2.0 or earlier, the following line has to be manually added to the