add documentation for ssha_password to sis csv upload
add documentation for ssha_password to sis csv upload

fixes CNVS-17612

test plan:
- try using this feature in sis csv import and make sure it works as
  advertised

Change-Id: I07517af1d9d8567642268057772c49cc47036eb5
Reviewed-on: https://gerrit.instructure.com/73329
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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