Makefile: document ground rules for target-specific dependencies

Makefile: document ground rules for target-specific dependencies

When a source file makes use of a makefile variable, there should be a
corresponding dependency on a file that changes when that variable
changes to ensure the build output is not left stale when the variable
changes.

Document this, even though we are not following the rule perfectly
yet.  Based on an explanation from Jeff King.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>