diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index a91fd214d2..6b930bc163 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -88,6 +88,16 @@ OPTIONS
 	sometimes helpful when working with a big project on a
 	filesystem that has very slow lstat(2) system call
 	(e.g. cifs).
++
+This option can be also used as a coarse file-level mechanism
+to ignore uncommitted changes in tracked files (akin to what
+`.gitignore` does for untracked files).
+You should remember that an explicit 'git add' operation will
+still cause the file to be refreshed from the working tree.
+Git will fail (gracefully) in case it needs to modify this file
+in the index e.g. when merging in a commit;
+thus, in case the assumed-untracked file is changed upstream,
+you will need to handle the situation manually.
 
 -g::
 --again::
diff --git a/Documentation/gitignore.txt b/Documentation/gitignore.txt
index fc0efd8ec8..59321a2e82 100644
--- a/Documentation/gitignore.txt
+++ b/Documentation/gitignore.txt
@@ -13,9 +13,14 @@ DESCRIPTION
 -----------
 
 A `gitignore` file specifies intentionally untracked files that
-git should ignore.  Each line in a `gitignore` file specifies a
-pattern.
-
+git should ignore.
+Note that all the `gitignore` files really concern only files
+that are not already tracked by git;
+in order to ignore uncommitted changes in already tracked files,
+please refer to the 'git update-index --assume-unchanged'
+documentation.
+
+Each line in a `gitignore` file specifies a pattern.
 When deciding whether to ignore a path, git normally checks
 `gitignore` patterns from multiple sources, with the following
 order of precedence, from highest to lowest (within one level of