Documentation: clarify git-checkout -f, minor editing

"Force a re-read of everything" doesn't mean much to me.

Also some minor grammar fixes.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-checkout.txt

@@ -23,9 +23,9 @@ options, which will be passed to `git branch`.
 
 When <paths> are given, this command does *not* switch
 branches.  It updates the named paths in the working tree from
-the index file (i.e. it runs `git-checkout-index -f -u`), or a
-named commit.  In
-this case, `-f` and `-b` options are meaningless and giving
+the index file (i.e. it runs `git-checkout-index -f -u`), or
+from a named commit.  In
+this case, the `-f` and `-b` options are meaningless and giving
 either of them results in an error.  <tree-ish> argument can be
 used to specify a specific tree-ish (i.e. commit, tag or tree)
 to update the index for the given paths before updating the
@@ -38,7 +38,8 @@ OPTIONS
 	Quiet, supress feedback messages.
 
 -f::
-	Force a re-read of everything.
+	Proceed even if the index or the working tree differs
+	from HEAD.  This is used to throw away local changes.
 
 -b::
 	Create a new branch named <new_branch> and start it at