Now that 'bst track' is obsolete, change guidance to refer to the
replacement 'bst source track' instead.

Now that 'bst fetch' is obsolete, change guidance to refer to the
replacement 'bst source fetch' instead.

Fixup refs to 'bst track' and 'bst fetch'

See merge request !1086

Valentin David authored Jan 23, 2019 and Valentin David committed Jan 28, 2019

Fixes #882.