doc: announce ethdev ABI change for MTU limits info

Maximum and minimum MTU values vary between hardware devices. In
hardware agnostic DPDK applications access to such information would
allow a more accurate way of validating and setting supported MTU values on
a per device basis rather than using a defined default for all devices.

The following solution was proposed:

http://mails.dpdk.org/archives/dev/2018-September/110959.html

This patch adds a depreciation notice for ``rte_eth_dev_info`` as new
members will be added to represent min and max MTU values. These can be
added to fit a hole in the existing structure for amd64 but not for 32 bit,
as such ABI change will occur as size of the structure will be impacted.

Signed-off-by: Ian Stokes <ian.stokes@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>