technical/shallow: stop referring to grafts

Now that grafts are deprecated, we should start to assume that readers
have no idea what grafts are. So it makes more sense to make the
description of the "shallow" feature stand on its own.

Suggested-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/shallow.txt

@@ -8,15 +8,10 @@ repo, and therefore grafts are introduced pretending that
 these commits have no parents.
 *********************************************************
 
-The basic idea is to write the SHA-1s of shallow commits into
-$GIT_DIR/shallow, and handle its contents like the contents
-of $GIT_DIR/info/grafts (with the difference that shallow
-cannot contain parent information).
-
-This information is stored in a new file instead of grafts, or
-even the config, since the user should not touch that file
-at all (even throughout development of the shallow clone, it
-was never manually edited!).
+$GIT_DIR/shallow lists commit object names and tells Git to
+pretend as if they are root commits (e.g. "git log" traversal
+stops after showing them; "git fsck" does not complain saying
+the commits listed on their "parent" lines do not exist).
 
 Each line contains exactly one SHA-1. When read, a commit_graft
 will be constructed, which has nr_parent < 0 to make it easier