[1.8.x] Documented gdal DataSource encoding parameter
[1.8.x] Documented gdal DataSource encoding parameter

Thanks Max Demars for the suggestion and Tim Graham for the review.
Backport of a8991b9b9f1 from master.

h4r5h1t/django-hauthy,pquentin/django,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,pquentin/django,pquentin/django,h4r5h1t/django-hauthy