Documentation fix and small enhancement in to_lower/2.
Documentation fix and small enhancement in to_lower/2.

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