Edit network analysis section of PyQGIS cookbook

Mainly fixed up some typos and grammar issues. Also added in details about what
values should be used in the QgsLineVectorLayerDirector class. The types
of input that are expected and what integer values are used with the
defaultDirection parameter.
Merge remote-tracking branch 'origin/fixup_netan' into fixup_netan

Conflicts:
	source/docs/pyqgis_developer_cookbook/network_analysis.rst