Document --addsuffix option in manual

docs/insert.md

@@ -29,11 +29,13 @@ If the genome has existing annotations in a GFF3 feature table, this can be
 supplied to the `--featuregff` option. The coordinates of the annotations will
 be updated after the insertion of IESs, and other annotation fields remain
 unchanged. If a feature is interrupted by one or more IESs, the feature will be
-split into two or more features, where the ID of the original feature is
-suffixed with `.seg_0`, `.seg_1` et seq. to distinguish them. The updating of
-an existing feature table is only available in insert mode, because the
-deletion or truncation of an existing feature annotation will require manual
-intervention or curation.
+split into two or more features. If the feature contains an ID attribute and
+the option `--addsuffix` is chosen, then the ID of the original feature is
+suffixed with `.seg_0`, `.seg_1` et seq. to distinguish the different segments
+split from the original entry. By default the ID field is unchanged. The
+updating of an existing feature table is only available in insert mode, because
+the deletion or truncation of an existing feature annotation will require
+manual intervention or curation.
 
 Output
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