docs: give more hints about how "add -e" works

docs: give more hints about how "add -e" works

The previous text was not exactly accurate; it is OK to
change space and minus lines, but only in certain ways. This
patch attempts to cover explicitly what can be done at the
individual line level, and cautions the user that
conceptually larger changes (like modifying a line) require
some understanding of the patch format.

Signed-off-by: Jeff King <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>