Add documentation for the tiny printf implementation used in SPL, TPL,
and VPL when CONFIG_SPL_USE_TINY_PRINTF (or equivalent) is enabled.
Document the supported format specifiers, limitations, and the warning
about snprintf() not performing bounds checking in this implementation.
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>