message: docs: Update v1.9 release date to 04.04
diff: @@ -2121,7 +2121,9 @@ Because of the differences in document object models, performance considerations
  :!numbered:
  [[v1.9]]
  -=== v1.9 ^2018-??-??^
  +=== v1.9 ^2018-04-04^
  +
  +Maintenance release. Changes:
  * Specification changes:
  . `xml_document::load(const char*)` (deprecated in 1.5) now has `deprecated` attribute; use `xml_document::load_string` instead
  