Documentation: do not assume that n > 1 in <rev>~$n

Documentation: do not assume that n > 1 in <rev>~$n

We explained <rev>~<n> as <n>th generation grand-parent, but a reader got
confused by the "grand-" part when <n> is 1.

Reword it with "ancestor"; with the "generation" and "following only the
first parents" around there, what we try to describe should be clear
enough now.

Noticed-by: Luke Diamand <redacted>
Helped-by: Thomas Rast <redacted>
Helped-by: Andreas Ericsson <redacted>
Signed-off-by: Junio C Hamano <redacted>