Update CSV upload modal to point to CSV docs

The following discussion from !173965 (merged) should be addressed:

• @m_frankiewicz started a discussion: (+4 comments)

        'UserMapping|Use a CSV file to reassign contributions from placeholder users to existing group members. For more information, see %{linkStart}reassign contributions and memberships by CSV%{linkEnd}.',

  But I believe this docs don't exist yet, adding CSV support is currently in dev.

Once we have docs for UCM by CSV upload, app/assets/javascripts/members/placeholders/components/csv_upload_modal.vue should point to them.