Documentation: git-diff asciidoc table workaround.

The table facility was nice in rendering HTML but was disastrous
for man page.  Reword the text and do not use table for now.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-diff.txt

@@ -17,14 +17,16 @@ ent and the index file, or the index file and the working tree.
 The combination of what is compared with what is determined by
 the number of ents given to the command.
 
-`----------------`--------`-----------------------------`------------------
-Number of ents    Options  What's Compared               Underlying command
----------------------------------------------------------------------------
-0                 -        index file and working tree   git-diff-files
-1                 --cached ent and index file            git-diff-index
-1                 -        ent and working tree          git-diff-index
-2                 -        two ents                      git-diff-tree
----------------------------------------------------------------------------
+* When no <ent> is given, the working tree and the index
+  file is compared, using `git-diff-files`.
+
+* When one <ent> is given, the working tree and the named
+  tree is compared, using `git-diff-index`.  The option
+  `--cached` can be given to compare the index file and
+  the named tree.
+
+* When two <ent>s are given, these two trees are compared
+  using `git-diff-tree`.
 
 OPTIONS
 -------