checkdoc-permit-comma-termination-flag is a variable defined in `checkdoc.el'.
Its value is

  • This variable is safe as a file local variable if its value
    satisfies the predicate `booleanp'.

Non-nil means the first line of a docstring may end with a comma.
Ordinarily, a full sentence is required. This may be misleading when
there is a substantial caveat to the one-line description -- the comma
should be used when the first part could stand alone as a sentence, but
it indicates that a modifying clause follows.

You can customize this variable.