Doc: update information about Clang Code Model Plugin II
Doc: update information about Clang Code Model Plugin II

* Update the instructions on building clang.
* State the minimum version of clang that is necessary (3.6.2).
* Rearrange the "acquire clang entries", the least time-consuming method
  for acquiring clang comes first now.

Change-Id: Ie0a80fa507be20fd7c3f09930de03a88634cccce
Reviewed-by: Leena Miettinen <885fda02d52616fec24ed5fed9b3f0568fce3ead@theqtcompany.com>
Reviewed-by: Marco Bubke <5254e489b34ec8bbce36789100e3fc0f41fa62ef@theqtcompany.com>

martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator