docs: Replace `\ref` with `::`

The code-base is currently fairly inconsistent with its usage of the
`::` prefix and the `\ref`. This commit replaces any instances of `\ref`
with `::`.

Change-Id: I02c1b6e2caf0f333388f44ac51ede060aa0568c3
Signed-off-by: Chris Kay <chris.kay@arm.com>