Updated signatures to support sphinx domain

[jonathan343]

Note: Should be merged after botocore's #2880. Before this happens, tests are expected to fail.

Overview

We previously had all class and method documentation on the same service page, however, we implemented nested docs to decrease page size. In doing so, we broke the way sphinx handles cross-references since methods on individual pages (not in the py:class body) should have the full class name attached as explained here.

Examples

Before Nested Docs:

# Page: <service>.html
.. py:class:: Foo
   .. py:method:: bar()

Current:

# Page: <service>.html
.. py:class:: Foo.Client

# Page: <method>.html
.. py:method:: bar()

New:

# Page: <service>.html
.. py:class:: Foo.Client

# Page: <method>.html
.. py:method:: Foo.Client.bar()

[nateprewitt]

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