git: update documentation for --git-dir

git --git-dir <path> is a bit confusing and sometimes doesn't work as
the user would expect it to.

For example, if the user runs `git --git-dir=<path> status`, git
will not be able to figure out the work tree path on its own and
will assign the work tree to the user's current work directory.
When this assignment is wrong, then the output will not match the
user's expectations.

This patch updates the documentation to make it clearer.

Signed-off-by: Heba Waly <[email protected]>

Author: HebaWaly

Documentation/git.txt

@@ -110,10 +110,15 @@ foo.bar= ...`) sets `foo.bar` to the empty string which `git config
 	Do not pipe Git output into a pager.
 
 --git-dir=<path>::
-	Set the path to the repository. This can also be controlled by
-	setting the `GIT_DIR` environment variable. It can be an absolute
-	path or relative path to current working directory.
-
+	Set the path to the repository (i.e. the .git folder). This can also be
+	controlled by setting the `GIT_DIR` environment variable. It can be
+	an absolute path or relative path to current working directory.
+
+	Note that --git-dir=<path> is not the same as -C=<path>.
+	It's preferrable to set --work-tree=<path> as well when setting
+	--git-dir to make sure Git will run your command across the correct
+	work tree.
+	
 --work-tree=<path>::
 	Set the path to the working tree. It can be an absolute path
 	or a path relative to the current working directory.