consolidate and enhance doc for vars CI_PROJECT_CONFIG_PATH and CI_CONFIG_PATH

Problem to solve

Please add more info to the documentation about the differences of CI_PROJECT_CONFIG_PATH and CI_CONFIG_PATH or consolidate them, depending on the outcome of the discussion at: #246802 (comment 506067774)

Plz, also mention that < v13.9 it's not possible to set these VARs dynamically using the trigger API. see #27987 (comment 506048292)

Further details

doc page: https://docs.gitlab.com/ee/ci/variables/predefined_variables.html