doc: Add more known-warnings

We should probably clear out the old warnings and see what still
applies for docleaf now that we have that instead of breathe.

However for this moment, these are new duplication declarations that
we're seeing errors for and so we'd like to silence them.

Signed-off-by: Michael Jones <[email protected]>

Author: michaeljones

doc/known-warnings.txt

@@ -16,3 +16,6 @@
 .*Duplicate C declaration.*\n.*'\.\. c:.*:: .*struct in6_addr.*'.*
 .*Duplicate C declaration.*\n.*'\.\. c:.*:: .*struct net_if.*'.*
 .*Duplicate C declaration.*\n.*'\.\. c:struct:: bt_ots_init'.*
+.*Duplicate C declaration.*\n.*'\.\. c:member:: enum *bt_mesh_dfd_upload_phase bt_mesh_dfd_srv.phase'.*
+.*Duplicate C declaration.*\n.*'\.\. c:member:: struct *bt_mesh_blob_xfer bt_mesh_dfu_cli.blob'.*
+.*Duplicate C declaration.*\n.*'\.\. c:member:: struct *net_if *\* net_if_mcast_monitor.iface'.