Documentation: reset: add some missing tables

and while at it also explain why --merge option is disallowed in some
cases.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-reset.txt

@@ -79,6 +79,13 @@ git reset --option target
 to reset the HEAD to another commit (`target`) with the different
 reset options depending on the state of the files.
 
+In these tables, A, B, C and D are some different states of a
+file. For example, the first line of the first table means that if a
+file is in state A in the working tree, in state B in the index, in
+state C in HEAD and in state D in the target, then "git reset --soft
+target" will put the file in state A in the working tree, in state B
+in the index and in state D in HEAD.
+
       working index HEAD target         working index HEAD
       ----------------------------------------------------
        A       B     C    D     --soft   A       B     D
@@ -107,12 +114,28 @@ reset options depending on the state of the files.
 				--hard   C       C     C
 				--merge  C       C     C
 
-In these tables, A, B, C and D are some different states of a
+      working index HEAD target         working index HEAD
-file. For example, the last line of the last table means that if a
+      ----------------------------------------------------
-file is in state B in the working tree and the index, and in a
+       B       C     C    D     --soft   B       C     D
-different state C in HEAD and in the target, then "git reset
+				--mixed  B       D     D
---merge target" will put the file in state C in the working tree,
+				--hard   D       D     D
-in the index and in HEAD.
+				--merge (disallowed)
+
+      working index HEAD target         working index HEAD
+      ----------------------------------------------------
+       B       C     C    C     --soft   B       C     C
+				--mixed  B       C     C
+				--hard   C       C     C
+				--merge  B       C     C
+
+"reset --merge" is meant to be used when resetting out of a conflicted
+merge. Any mergy operation guarantees that the work tree file that is
+involved in the merge does not have local change wrt the index before
+it starts, and that it writes the result out to the work tree. So if
+we see some difference between the index and the target and also
+between the index and the work tree, then it means that we are not
+resetting out from a state that a mergy operation left after failing
+with a conflict. That is why we disallow --merge option in this case.
 
 The following tables show what happens when there are unmerged
 entries: