doc: sandbox: Add additional valgrind documentation

doc: sandbox: Add additional valgrind documentation

This documents some additional options which can be used with valgrind, as
well as directions for future work. It also fixes up inline literals to
actually be inline literals (and not italics). The content of this
documentation is primarily adapted from [1].

[1] https://lore.kernel.org/u-boot/57cb4b49-fa30-1194-9ac3-faa53e8033bd@gmail.com/

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>