technical/shallow: describe why shallow cannot use replace refs

It is tempting to do away with commit_graft altogether (in the long
haul), now that grafts are deprecated.

However, the shallow feature needs a couple of things that the replace
refs cannot fulfill. Let's point that out in the documentation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/shallow.txt

@@ -17,6 +17,13 @@ Each line contains exactly one SHA-1. When read, a commit_graft
 will be constructed, which has nr_parent < 0 to make it easier
 to discern from user provided grafts.
 
+Note that the shallow feature could not be changed easily to
+use replace refs: a commit containing a `mergetag` is not allowed
+to be replaced, not even by a root commit. Such a commit can be
+made shallow, though. Also, having a `shallow` file explicitly
+listing all the commits made shallow makes it a *lot* easier to
+do shallow-specific things such as to deepen the history.
+
 Since fsck-objects relies on the library to read the objects,
 it honours shallow commits automatically.