Clarified module handling of set_prolog_flag/2.
Clarified module handling of set_prolog_flag/2.

jn7163/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel