Documentation: Add usage example for verbs import

Add a documentation for shared PD and verbs import usage in Pyverbs.
This includes a code snippet to demonstrate a usage example.

Reviewed-by: Ido Kalir <idok@mellanox.com>
Signed-off-by: Edward Srouji <edwards@mellanox.com>

Documentation/pyverbs.md

@@ -626,3 +626,43 @@ ctx = Context(name='rocep0s8f0')
 uar = Mlx5UAR(ctx)
 uar.close()
 ```
+
+##### Import device, PD and MR
+Importing a device, PD and MR enables processes to share their context and then
+share PDs and MRs that is associated with.
+A process creates a device and then uses some of the Linux systems calls to dup
+its 'cmd_fd' member which lets other process to obtain ownership.
+Once other process obtains the 'cmd_fd' it can import the device, then PD(s) and
+MR(s) to share these objects.
+Like in C, Pyverbs users are responsible for unimporting the imported objects
+(which will also close the Pyverbs instance in our case) after they finish using
+them, and they have to sync between the different processes in order to
+coordinate the closure of the objects.
+Unlike in C, closing the underlying objects is currently supported only via the
+"original" object (meaning only by the process that creates them) and not via
+the imported object. This limitation is made because currently there's no
+reference or relation between different Pyverbs objects in different processes.
+But it's doable and might be added in the future.
+Here is a demonstration of importing a device, PD and MR in one process.
+```python
+from pyverbs.device import Context
+from pyverbs.pd import PD
+from pyverbs.mr import MR
+import pyverbs.enums as e
+import os
+
+ctx = Context(name='ibp0s8f0')
+pd = PD(ctx)
+mr = MR(pd, 100, e.IBV_ACCESS_LOCAL_WRITE)
+cmd_fd_dup = os.dup(ctx.cmd_fd)
+improted_ctx = Context(cmd_fd=cmd_fd_dup)
+imported_pd = PD(improted_ctx, handle=pd.handle)
+imported_mr = MR(imported_pd, handle=mr.handle)
+# MRs can be created as usual on the imported PD
+secondary_mr = MR(imported_pd, 100, e.IBV_ACCESS_REMOTE_READ)
+# Must manually unimport the imported objects (which close the object and frees
+# other resources that use them) before closing the "original" objects.
+# This prevents unexpected behaviours caused by the GC.
+imported_mr.unimport()
+imported_pd.unimport()
+```