docs/build: Avoid using multi-target pattern rules

docs/build: Avoid using multi-target pattern rules

Multi-target non-pattern rules and Multi-target pattern rules behave rather
differently.  From `Pattern Intro':

  Pattern rules may have more than one target.  Unlike normal rules, this does
  not act as many different rules with the same prerequisites and commands.
  If a pattern rule has multiple targets, `make' knows that the rule's
  commands are responsible for making all of the targets.  The commands are
  executed only once to make all the targets.

The intended use of the multi-target pattern rules was to avoid repeating the
identical recipe multiple times.  The issue can be demonstrated with the
generation of documentation from pandoc source.

  ./xen.git$ touch docs/features/template.pandoc
  ./xen.git$ make -C docs/
  # Regenerates html/features/template.html
  ./xen.git$ make -C docs/
  # Regenerates txt/features/template.txt
  ./xen.git$ make -C docs/
  # Regenerates pdf/features/template.pdf

To work around this, there need to be three distinct rules, so the execution
of one recipe doesn't short ciruit the others.  To avoid copy&paste
duplication, introduce a metarule, and evalute it for each document target.

As $(PANDOC) is used to generate documentation from different source types,
the metarule can be extended to also encompas the rule to create pdfs from
markdown.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>