directory-rename-detection.txt: update references to regression tests

The regression tests for directory rename detection were renamed from
t6043 to t6423 in commit 919df31955 ("Collect merge-related tests to
t64xx", 2020-08-10); update this file to match.  Also, add a small
clarification to nearby text while we're at it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/directory-rename-detection.txt

@@ -18,7 +18,8 @@ It is perhaps easiest to start with an example:
 More interesting possibilities exist, though, such as:
 
   * one side of history renames x -> z, and the other renames some file to
-    x/e, causing the need for the merge to do a transitive rename.
+    x/e, causing the need for the merge to do a transitive rename so that
+    the rename ends up at z/e.
 
   * one side of history renames x -> z, but also renames all files within x.
     For example, x/a -> z/alpha, x/b -> z/bravo, etc.
@@ -35,7 +36,7 @@ More interesting possibilities exist, though, such as:
     directory itself contained inner directories that were renamed to yet
     other locations).
 
-  * combinations of the above; see t/t6043-merge-rename-directories.sh for
+  * combinations of the above; see t/t6423-merge-rename-directories.sh for
     various interesting cases.
 
 Limitations -- applicability of directory renames
@@ -62,7 +63,7 @@ directory rename detection applies:
 Limitations -- detailed rules and testcases
 -------------------------------------------
 
-t/t6043-merge-rename-directories.sh contains extensive tests and commentary
+t/t6423-merge-rename-directories.sh contains extensive tests and commentary
 which generate and explore the rules listed above.  It also lists a few
 additional rules: