Document that per-file lines should not contain additional spaces

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Idaafa1784903b3c346c18a1f2fd9b1ae3e8b4a48

resources/Documentation/backend-find-owners.md

diff --git a/resources/Documentation/backend-find-owners.md b/resources/Documentation/backend-find-owners.md
index 9d2fab5..223cebb 100644
--- a/resources/Documentation/backend-find-owners.md
+++ b/resources/Documentation/backend-find-owners.md
@@ -266,6 +266,13 @@
   per-file docs.config,*.md=richard.roe@example.com
 ```
 
+**NOTE:** It's important to not include additional spaces in `per-file` lines.
+E.g. "per-file docs.config, test.config=richard.roe@example.com" will make
+Richard Roe a code owner of the files "docs.config" and " test.config" (pay
+attention to the leading space), but not for the file "test.config". The correct
+line to make Richard Roe a code owner of the files "docs.config" and
+"test.config" would be "per-file docs.config,test.config=richard.roe@example.com".
+
 ##### <a id="doNotUsePathExpressionsForSubdirectories">
 **NOTE:** It is discouraged to use path expressions that explicitly name
 subdirectories such as `my-subdir/*` as they will break when the subdirectory