docs(uxrce_dds): add docs for UXRCE_DDS_FLCTRL (PX4#26242)

Signed-off-by: Beniamino Pozzan <beniamino.pozzan@gmail.com>

Author: beniaminopozzan

docs/en/middleware/uxrce_dds.md

@@ -321,9 +321,11 @@ The configuration can be done using the [UXRCE-DDS parameters](../advanced_confi
   - [UXRCE_DDS_SYNCT](../advanced_config/parameter_reference.md#UXRCE_DDS_SYNCT): Bridge time synchronization enable.
     The uXRCE-DDS client module can synchronize the timestamp of the messages exchanged over the bridge.
     This is the default configuration. In certain situations, for example during [simulations](../ros2/user_guide.md#ros-gazebo-and-px4-time-synchronization), this feature may be disabled.
-  - [UXRCE_DDS_NS_IDX](../advanced_config/parameter_reference.md#UXRCE_DDS_NS_IDX) <Badge type="tip" text="PX4 v1.17" />: Index-based namespace definition
+  - [UXRCE_DDS_NS_IDX](../advanced_config/parameter_reference.md#UXRCE_DDS_NS_IDX) <Badge type="tip" text="PX4 v1.17" />: Index-based namespace definition.
     Setting this parameter to any value other than `-1` creates a namespace with the prefix `uav_` and the specified value, e.g. `uav_0`, `uav_1`, etc.
     See [namespace](#customizing-the-namespace) for methods to define richer or arbitrary namespaces.
+  - [`UXRCE_DDS_FLCTRL`](../advanced_config/parameter_reference.md#UXRCE_DDS_FLCTRL) <Badge type="tip" text="PX4 main" />: Serial port hardware flow control enable.
+    To use hardware flow control, a custom MicroXRCE Agent needs to be adopted. Please refer to [this PR](https://github.com/eProsima/Micro-XRCE-DDS-Agent/pull/407) for the required changes, cherry-pick them on top of the [agent version](#build-run-within-ros-2-workspace) you need to use and then run the agent with the additional `--flow-control` option.
 
 ::: info
 Many ports are already have a default configuration.