Documentation: reset: add some tables to describe the different options

This patch adds a DISCUSSION section that contains some tables to
show how the different "git reset" options work depending on the
states of the files in the working tree, the index, HEAD and the
target commit.

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

Documentation/git-reset.txt

@@ -67,6 +67,72 @@ linkgit:git-add[1]).
 <commit>::
 	Commit to make the current HEAD. If not given defaults to HEAD.
 
+DISCUSSION
+----------
+
+The tables below show what happens when running:
+
+----------
+git reset --option target
+----------
+
+to reset the HEAD to another commit (`target`) with the different
+reset options depending on the state of the files.
+
+      working index HEAD target         working index HEAD
+      ----------------------------------------------------
+       A       B     C    D     --soft   A       B     D
+				--mixed  A       D     D
+				--hard   D       D     D
+				--merge (disallowed)
+
+      working index HEAD target         working index HEAD
+      ----------------------------------------------------
+       A       B     C    C     --soft   A       B     C
+				--mixed  A       C     C
+				--hard   C       C     C
+				--merge (disallowed)
+
+      working index HEAD target         working index HEAD
+      ----------------------------------------------------
+       B       B     C    D     --soft   B       B     D
+				--mixed  B       D     D
+				--hard   D       D     D
+				--merge  D       D     D
+
+      working index HEAD target         working index HEAD
+      ----------------------------------------------------
+       B       B     C    C     --soft   B       B     C
+				--mixed  B       C     C
+				--hard   C       C     C
+				--merge  C       C     C
+
+In these tables, A, B, C and D are some different states of a
+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
+different state C in HEAD and in the target, then "git reset
+--merge target" will put the file in state C in the working tree,
+in the index and in HEAD.
+
+The following tables show what happens when there are unmerged
+entries:
+
+      working index HEAD target         working index HEAD
+      ----------------------------------------------------
+       X       U     A    B     --soft  (disallowed)
+				--mixed  X       B     B
+				--hard   B       B     B
+				--merge (disallowed)
+
+      working index HEAD target         working index HEAD
+      ----------------------------------------------------
+       X       U     A    A     --soft  (disallowed)
+				--mixed  X       A     A
+				--hard   A       A     A
+				--merge (disallowed)
+
+X means any state and U means an unmerged index.
+
 Examples
 --------