Fixes #1916

The class docstring for FetcherInterface needed to clearly state that
only _fetch() had to be implemented in it's implementation. This is
because the public API of the interface is implemented already.

Signed-off-by: Abhisman Sarkar <abhisman.sarkar@gmail.com>

Author: abs007

docs/api/tuf.ngclient.fetcher.rst

@@ -3,3 +3,4 @@ Fetcher
 
 .. automodule:: tuf.ngclient.fetcher
    :undoc-members:
+   :private-members: _fetch

tuf/ngclient/fetcher.py

@@ -23,6 +23,9 @@ class FetcherInterface:
     By providing a concrete implementation of the abstract interface,
     users of the framework can plug-in their preferred/customized
     network stack.
+
+    Implementations of FetcherInterface only need to implement ``_fetch()``.
+    The public API of the class is already implemented.
     """
 
     __metaclass__ = abc.ABCMeta