Update unmerge docs
# Unmerge
+`unmerge` removes all axioms contained in one ontology from another. The first `--input` is the ontology to remove axioms from, subsequent `--input` ontologies contain the axioms to be removed:
+
+<!-- DO NOT TEST -->
+```
+robot unmerge --input my-ontology.owl \
+ --input remove-axioms.owl \
+ --input remove-axioms-2.owl \
+ --output my-ontology-fixed.owl
+```
+
+If any axioms exist in the subsequent ontologies that do not exist in the first ontology, they will be silently ignored.
+
+### Unmerging a Merge
+
A [merge](/merge) can be 'undone' using the unmerge command. This can be useful if we want to perform some operation (for example, reasoning) on a merged file, but later subtract the results.
This will merge in foo, and then subtract out foo from the merged ontology.
