Docs update

docs/query_assignment.rst

@@ -205,7 +205,11 @@ input folder will contain the updated database containing everything needed.
 .. note::
     This mode can take longer to run with large numbers of input query genomes,
     as it will calculate all :math:`Q^2` query-query distances, rather than
-    just those found in novel query clusters.
+    just those found in novel query clusters. Furthermore, you may observe query genomes previously
+    assigned to novel clusters without ``--update-db`` being assigned to existing clusters when using
+    this option. This is expected behaviour, and is a manifestation of cluster merging, whereby the comparison
+    of all database genomes to queries, not just references, enables queries to be assigned to existing clusters.
+    See :doc:`troubleshooting` for more details.
 
 Visualising results
 -------------------

docs/troubleshooting.rst

@@ -38,6 +38,25 @@ as above.
 
 See `issue 194 <https://github.com/bacpop/PopPUNK/issues/194>`__ for more discussion.
 
+I get different cluster assignments for my queries with and without --update-db flag
+------------------------------------------------------------------------------------
+
+When using ``poppunk_assign``, you may observe query genomes being assigned to novel clusters when running
+without the ``--update-db`` option. However, using ``--update-db`` with the same query genomes may result 
+in assignment of queries to existing clusters, rather than to novel clusters, making cluster assignments seem inconsistent.
+
+However, this is expected behaviour. When running ``poppunk_assign`` without ``--update-db``, query genomes are compared only to
+eachother and to reference sequences in the database to make this step efficient. In this instance, the query genomes may be too diverged
+from any existing references to cluster with them, and therefore will be assigned to a novel cluster. 
+
+When running ``poppunk_assign`` with ``--update-db``, query genomes are compared to all genomes in the database, not just
+references. The larger number of comparisons means that the same queries may cluster with non-reference genomes, which 
+themselves cluster with a references genome. Therefore, the queries are linked indirectly to a reference genome in the distance
+network, meaning they are assigned to an existing cluster, not a novel one as before.
+
+We recommend users who find 'novel' clusters in their datasets when running ``poppunk_assign`` without ``--update-db`` also check
+against results with ``--update-db`` to determine whether the clusters are truly novel, or form part of existing clusters in the full database.
+
 Memory/run-time issues
 ----------------------
 Here are some tips based on experiences analysing larger datasets:
@@ -156,3 +175,4 @@ If you want to change cluster names or assign queries to your own cluster defini
 you can use the ``--external-clustering`` argument instead.
 
 
+