git-commit.txt - mention that files listed on the command line must be known to git.

Signed-off-by: Mark Burton <markb@ordern.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: burto

Documentation/git-commit.txt

@@ -29,7 +29,8 @@ The content to be added can be specified in several ways:
 
 3. by listing files as arguments to the 'commit' command, in which
    case the commit will ignore changes staged in the index, and instead
-   record the current content of the listed files;
+   record the current content of the listed files (which must already
+   be known to git);
 
 4. by using the -a switch with the 'commit' command to automatically
    "add" changes from all known files (i.e. all files that are already