doc: keep first level section header in upper case

When formatted as a man page, 1st section header is always in upper
case even if we write it otherwise. Make all 1st section headers
uppercase to keep it close to the final output.

This does affect html since case is kept there, but I still think it's
a good idea to maintain a consistent style for 1st section headers.

Some sections perhaps should become second sections instead, where
case is kept, and for better organization. I will update if anyone has
suggestions about this.

While at there I also make some header more consistent (e.g. examples
vs example) and fix a couple minor things here and there.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-diff-index.txt b/Documentation/git-diff-index.txt
index b380677..f4bd815 100644
--- a/Documentation/git-diff-index.txt
+++ b/Documentation/git-diff-index.txt
@@ -37,14 +37,14 @@
 
 include::diff-format.txt[]
 
-Operating Modes
+OPERATING MODES
 ---------------
 You can choose whether you want to trust the index file entirely
 (using the `--cached` flag) or ask the diff logic to show any files
 that don't match the stat state as being "tentatively changed".  Both
 of these operations are very useful indeed.
 
-Cached Mode
+CACHED MODE
 -----------
 If `--cached` is specified, it allows you to ask:
 
@@ -77,7 +77,7 @@
 asking yourself "what have I already marked for being committed, and
 what's the difference to a previous tree".
 
-Non-cached Mode
+NON-CACHED MODE
 ---------------
 The "non-cached" mode takes a different approach, and is potentially
 the more useful of the two in that what it does can't be emulated with