Add launch_type options and examples to sessionless launch
Add launch_type options and examples to sessionless launch

Prior to this update, it was unclear what options were availble for
launch_type. There were also not examples for the request. This update adds
 examples and clarifies what values for launch_type are acceptible.

Test plan:
1) run localhost Canvas
2) build the API docs (docker-compose run --rm web bundle exec rake
doc:api)
3) navigate to <your host url>/doc/api/external_tools.html
4) check for spelling, grammar, accuracy of information and that the new
link works

Fixes PLAT-1735

Change-Id: I348cadf57b196b710cd96e700183a305a9a550ce
Reviewed-on: https://gerrit.instructure.com/93412
Tested-by: Jenkins
Reviewed-by: Nathan Mills <10b0771d20c18ad0d98c1d000665e443052b40b0@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Karl Lloyd <4c8060af93345c2cc416ee6aa55bceec4e5cc9d4@instructure.com>

djbender/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms