Change section title in doc

From OGRFieldDefn to OGRFeatureDefn

On branch master
Add geom type function for feature defn

* fd_get_geom_type(feature_defn)

On branch master
Add fld_get_type function for field defn.

On branch master
Add fld_get_name_ref function for field defn.

On branch master
Add fd_get_field_count function.

DataSource = erlogr:open("test/polygon.shp"),
Layer = erlogr:ds_get_layer(DataSource, 0),
FeatureDefn = erlogr:l_get_layer_defn(Layer),
FieldCount = erlogr:fd_get_field_count(FeatureDefn).

On branch master