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/plat/meson-g12a.rst b/docs/plat/meson-g12a.rst
index 1021da4..7cd1bf7 100644
--- a/docs/plat/meson-g12a.rst
+++ b/docs/plat/meson-g12a.rst
@@ -1,5 +1,5 @@
 Amlogic Meson S905X2 (G12A)
-==========================
+===========================
 
 The Amlogic Meson S905X2 is a SoC with a quad core Arm Cortex-A53 running at
 ~1.8GHz. It also contains a Cortex-M3 used as SCP.