doc: Misc syntax and spelling fixes

Tidying up a few Sphinx warnings that had built-up over time.
None of these are critical but it cleans up the Sphinx output.

At the same time, fixing some spelling errors that were detected.

Change-Id: I38209e235481eed287f8008c6de9dedd6b12ab2e
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/security_advisories/security-advisory-tfv-6.rst b/docs/security_advisories/security-advisory-tfv-6.rst
index 495eddd..9eeaeec 100644
--- a/docs/security_advisories/security-advisory-tfv-6.rst
+++ b/docs/security_advisories/security-advisory-tfv-6.rst
@@ -51,7 +51,7 @@
 For Cortex-A73 and Cortex-A75 CPUs, the PRs in this advisory invalidate the
 branch predictor when entering EL3 by temporarily dropping into AArch32
 Secure-EL1 and executing the ``BPIALL`` instruction. This workaround is
-signifiantly more complex than the "MMU disable/enable" workaround. The latter
+significantly more complex than the "MMU disable/enable" workaround. The latter
 is not effective at invalidating the branch predictor on Cortex-A73/Cortex-A75.
 
 Note that if other privileged software, for example a Rich OS kernel, implements