Take for example "-keep".  I do 'C-h v -keep RET' and get:

  -keep is void as a variable.

  Not documented as a variable.

Now I eval (require 'dash), do 'C-h v -keep RET' again and get:

  -keep is void as a variable.

  Not documented as a variable.

whereas 'C-h f -keep' returns:

  -keep is a native-comp-function in ‘dash.el’.

  (-keep FN LIST)

  Return a new list of the non-nil results of applying FN to each item in LIST.
  Like ‘-filter’, but returns the non-nil results of FN instead of
  the corresponding elements of LIST.

  Its anaphoric counterpart is ‘--keep’.

In summary, I see a void variable in the list of completions where the
function is defined.  Also, when I do 'emacs -Q' followed directly with
'C-h f TAB', I see this: