Added instructions for alternate means to pin a discussion .

Fixes CNVS-11971

Test plan:
* Visit a discussions listing page.
* Unpin all discussions, if any.
* Verify that the instructions on how to pin a discussion include information
  on how to do so without requiring the use of a mouse.
* Verify that the instructions are clear and correct.

Change-Id: Ic330b21cbbc4645c0c44bc5cc41442a8519170b3
Reviewed-on: https://gerrit.instructure.com/32391
Product-Review: Jon Willesen <jonw@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
Product-Review: Dana Danger <dana@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>