docbook: use IDs as filenames to support multiple books

I'm trying to generate a <set> of <book>s in docbook for wireless to
link together all the cfg80211 and mac80211 documentation.

However, docbook will generate "re01.html" anew for each book for the
first <refentry>, presumably due to a bug in the stylesheets.

An effective workaround is to use IDs for the filenames, which makes
them more descriptive as well, e.g.  API-enum-ieee80211-band.html.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/stylesheet.xsl

@@ -6,4 +6,5 @@
 <param name="callout.graphics">0</param>
 <!-- <param name="paper.type">A4</param> -->
 <param name="generate.section.toc.level">2</param>
+<param name="use.id.as.filename">1</param>
 </stylesheet>