[PATCH 2/2] docs: checkpatch: Add the description for RFC_TAG_PREFIX waining messsage

[Cai Huoqing]

A check item for RFC tags is added to checkpatch.pl,
which will report RFC_TAG_PREFIX warning. so add
the description for RFC_TAG_PREFIX.

Signed-off-by: Cai Huoqing <caihuoqing@xxxxxxxxx>
---
 Documentation/dev-tools/checkpatch.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/dev-tools/checkpatch.rst b/Documentation/dev-tools/checkpatch.rst
index b52452bc2963..0da827af3d93 100644
--- a/Documentation/dev-tools/checkpatch.rst
+++ b/Documentation/dev-tools/checkpatch.rst
@@ -554,6 +554,16 @@ Commit message
 
     See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
 
+  **RFC_TAG_PREFIX**
+    RFC tag is outside subject prefix.
+    RFC tag outside subject prefix is incorrect like this::
+
+      Subject: [PATCH v2] RFC:
+
+    Perfer::
+
+      Subject: [RFC PATCH v2]
+
   **FROM_SIGN_OFF_MISMATCH**
     The author's email does not match with that in the Signed-off-by:
     line(s). This can be sometimes caused due to an improperly configured
-- 
2.25.1