docs: Prefer `\` to `@` in Doxygen comments

This commit replaces any instances of `@`-style Doxygen commands with
their `\`-style equivalents. This is done for consistency across the
codebase.

The `\sa` Doxygen command has been added to the Clang-Format
configuration to prevent it from reflowing see-also comments, which
often exceed the column limit due to referring to long URLs.

Change-Id: Ia4ebe8bd310293da9e2af55fe253f0762807e88a
Signed-off-by: Chris Kay <chris.kay@arm.com>