docs: Noted that JobIntentService is usually a better choice

Since IntentService is subject to the O background restrictions,
most devs are better off switching to the new JobIntentService.
(I assume IntentService is not actually deprecated; if it is, tell
me and I'll change this to a @deprecated tag!)

See first comment for doc stage location.

Test: make ds-docs
Bug: 64159987
Change-Id: I83a53d1e6336c2134bf4c61bedd2ae42cd80493a