Reading the manual it does not seem to be anything bad. MySQL just informs you about an additional action it had to do.
InnoDB uses a unique document identifier referred to as a Document ID
(DOC_ID) to map words in the full-text index to document records where
the word appears. The mapping requires an FTS_DOC_ID column on the
indexed table. If an FTS_DOC_ID column is not defined, InnoDB
automatically adds a hidden FTS_DOC_ID column when the full-text index
If you preferred to not get the warning, then you would have to manage the column yourself which looks like a lot of work which is currently automated:
If you choose to define the FTS_DOC_ID column yourself, you are
responsible for managing the column to avoid empty or duplicate
values. FTS_DOC_ID values cannot be reused, which means FTS_DOC_ID
values must be ever increasing.
One side effect of the actions producing this warning is that the table has to be rebuild - that might take some time for tables already containing many records. But the same will be true if you added the column yourself, so it does not make a difference for you.