doc: cmd: wget: document lwIP syntax

doc: cmd: wget: document lwIP syntax

The lwIP version of wget supports a different syntax with a URL,
in addition to the legacy syntax. Document that.

While we're at it, fix a couple of minor issues in the legacy
syntax:
- hostIPaddr can be a DNS name if CONFIG_CMD_DNS is enabled
- path is mandatory

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>