tools/qa-checks: Improve the spelling QA check

This patch improves the spelling QA check to make the check less likely to
produce false-positives, and to reduce the maintainence effort on the internal
custom dictionary.

This patch:
* Avoids 'spelling errors' found within ReStructuredText code-blocks or links,
  as these are almost certainly going to be technical words that do not need to
  be validated against standard English spelling. Headers are still checked, so
  internal links are validated at the definition.
* Expand substrings before validation, to allow the custom dictionary to
  contain (and successfully pass) only proper words.

Issue-Id: SCM-3033
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I5303b93293d27c8b04116e19313c60790b5b1710