Bug#1006566: man should mention adb backup is deprecated and may be removed

[積丹尼 Dan Jacobson]

Package: adb
Version: 1:29.0.6-6
File: /usr/share/man/man1/adb.1.gz

Running adb backup says:
WARNING: adb backup is deprecated and may be removed in a future release

Therefore the adb backup section on the man page should mention this
fact also.