This patchset adds documentation on document encryption. It also
adds appropriate references to other sections of the documentation
where linking to document encryption is apropos. Finally, this
patchset adds further information and structure around the document
rendering process in general, in order to underscore the pieces
of documentation that undergird the general concept of document
rendering: substitution, layering and replacement.
Change-Id: I566cac4a3374556a0e62e0e1962e153029f7ec05
fb6220f1b5