Fortran allows specification expressions, i.e. non-constant scalar integer expressions, to specify array bounds. This may e.g. used to compute the length of a string returned as a function result. A function defined as:

  !> @brief The suffix of a file.
  FUNCTION suffix(filename)
    CHARACTER(len=*), INTENT(in) :: filename
    CHARACTER(len=LEN_TRIM(filename) - INDEX(filename, '.', .TRUE.)) :: suffix
    suffix = [...]
  END FUNCTION

results in this documentation snippet:
  CHARACTER(len=LEN_TRIM(filename)   suffix (filename)
                                     The suffix of a file. 

Here, the LEN=... expression is incomplete. The same holds for the detailed view when available.

Instead of replicating the full expression in the brief-list, I'd suggest to replace specification expressions with '...' (here, "CHARACTER(len=...) suffix") and only print the full expression in the detailed view as such expression may be arbitrarily long.