docs: IntelliJ. rm py-mode for BUILDs. re-order. detail for 'goal idea'

Inspired by some advice from Fedor Korotkov on the occasion of
his new IntelliJ plugin for Pants release

"Probably you should just move Pants Build Plugin for IntelliJ
 to the top or just remove Editing BUILD Files ;-)"

Reviewed at https://rbcommons.com/s/twitter/r/357/