diff --git a/src/calibre/ebooks/conversion/plugins/mobi_output.py b/src/calibre/ebooks/conversion/plugins/mobi_output.py
index b3577295e7..dbe52a5a08 100644
--- a/src/calibre/ebooks/conversion/plugins/mobi_output.py
+++ b/src/calibre/ebooks/conversion/plugins/mobi_output.py
@@ -56,12 +56,11 @@ class MOBIOutput(OutputFormatPlugin):
             help=_('Disable compression of the file contents.')
         ),
         OptionRecommendation(name='personal_doc', recommended_value='[PDOC]',
-            help=_('Tag marking book to be filed with Personal Docs.'
-                   ' If the book being converted has this tag in its metadata,'
-                   ' then the generated MOBI file'
-                   ' will be marked as a personal document. If you want to ensure'
-                   ' that the generated file is always marked as a personal document,'
-                   ' use the --tags option in addition.')
+            help=_('Tag for MOBI files to be marked as personal documents.'
+                   ' This option has no effect on the conversion. It is used'
+                   ' only when sending MOBI files to a device. If the file'
+                   ' being sent has the specified tag, it will be marked as'
+                   ' a personal document when sent to the Kindle.')
         ),
         OptionRecommendation(name='mobi_ignore_margins',
             recommended_value=False,