doc: remove "directory cache" from man pages

"directory cache" (or "directory cache index", "cache") are obsolete
terms which have been superseded by "index". Keeping them in the
documentation may be a source of confusion. This commit replaces
them with the current term, "index", on man pages.

Signed-off-by: Utku Gultopu <ugultopu@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-files.txt

@@ -23,9 +23,8 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-This merges the file listing in the directory cache index with the
-actual working directory list, and shows different combinations of the
-two.
+This merges the file listing in the index with the actual working
+directory list, and shows different combinations of the two.
 
 One or more of the options below may be used to determine the files
 shown:

Documentation/git-update-index.txt

@@ -30,9 +30,8 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Modifies the index or directory cache. Each file mentioned is updated
-into the index and any 'unmerged' or 'needs updating' state is
-cleared.
+Modifies the index. Each file mentioned is updated into the index and
+any 'unmerged' or 'needs updating' state is cleared.
 
 See also linkgit:git-add[1] for a more user-friendly way to do some of
 the most common operations on the index.