Streamlining/Correction of the changes made in r603926

Streamlining/Correction of the changes made in r603926
- delegate validation of the fo:wrapper's children to the parent: added static FONode.validateChildNode()
- narrow the condition for processing text-childnodes:
  this is not only constrained to fo:flow and fo:static-content, but the same goes
  for a fo:wrapper that is a direct descendant of a fo:block-container or fo:inline-container,
  which only allow block-level content (interpretation)
- minor javadoc fixups/improvements

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@604678 13f79535-47bb-0310-9956-ffa450edef68