Submit requirements: document file operator with content pattern

In change I8219135b we extended the file predicate as a submit
requirement predicate and added support to check on file modifications
as well. We did not document the new operator hence doing in this
change.

Change-Id: I3ae07b3f648802e9cb25b39f619154e51d43ee5a
Release-Notes: Support checking on content modifications with the file operator for submit requirements.
diff --git a/Documentation/config-submit-requirements.txt b/Documentation/config-submit-requirements.txt
index dfb033c..3e740a4 100644
--- a/Documentation/config-submit-requirements.txt
+++ b/Documentation/config-submit-requirements.txt
@@ -178,6 +178,12 @@
 redefine a submit requirement in a child project and make the submit requirement
 always non-applicable.
 
+[[operator_file]]
+file:"'<filePattern>',withDiffContaining='<contentPattern>'"::
++
+An operator that returns true if the latest patchset contained a modified file
+matching `<filePattern>` with a modified region matching `<contentPattern>`.
+
 [[unsupported_operators]]
 === Unsupported Operators