doc: Improve documentation wording, fix inconsistencies

This patch:
* Re-words and re-structures some documentation guidance to be more preicse
  about what is being referred to
* Makes FVP-Base consistently refer to the proper name of the FVP, and fvp-base
  refer to the Yocto MACHINE
* Fixes some inconsistent capitalisation
* Fixes some inconsistent structure/indentation

Issue-Id: SCM-3042
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Ibfd71fdf697fd6e8e1307755e9b38a1dbd8459df