[PATCH] doc: fix modify field action description for mlx5

[Viacheslav Ovsiienko <viacheslavo@nvidia.com>]

This patch adds mlx5 specifics description about
handling the Ethernet type by modify field action
for VLAN-ed traffic.

Fixes: 641dbe4fb053 ("net/mlx5: support modify field flow action")
Cc: stable@dpdk.org

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
---
 doc/guides/nics/mlx5.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst
index 7b04e9bac5..efc971a89e 100644
--- a/doc/guides/nics/mlx5.rst
+++ b/doc/guides/nics/mlx5.rst
@@ -407,6 +407,11 @@ Limitations
   - Modification of the 802.1Q Tag, VXLAN Network or GENEVE Network ID's is not supported.
   - Encapsulation levels are not supported, can modify outermost header fields only.
   - Offsets must be 32-bits aligned, cannot skip past the boundary of a field.
+  - if the field type is RTE_FLOW_FIELD_MAC_TYPE and packet contains one
+    or more VLAN headers, the meaningful type field following the the last
+    VLAN header is used as modify field operation argument. The modify field
+    action is not intended to modify VLAN headers type field, dedicated VLAN
+    push and pop actions should be used instead.
 
 - IPv6 header item 'proto' field, indicating the next header protocol, should
   not be set as extension header.
-- 
2.18.1