Updates to the template about policy changes

Updated the API section to add a note about policy changes.

This highlights an inconsistency in the section titles that I have
also addressed as part of this change. There are deployer impacts
mentioned in many sections outside the deployer impact section.

Ideas are taken from early revisions of:
https://review.openstack.org/#/c/82166

Change-Id: Ie2161adcfe378ce33e5406f35a2cf1a14fbcc25b
Make bullet lists more consistent

Chosen spaces and *, over condensed and -, because it seems to
look the most like bullets, and stops you forgetting the required
spaces for new sublists.

Change-Id: I8df88378372e3755bd440fcc2c96a4c7d4346b42
Move usage notes back into body of template

When you render this template, we currently exclude the usage notes.
While this reduces the amount of text to be deleted, it really seems
quite unhelpful when viewing this in github or the tox generated
html.

Change-Id: Ib0acf5cf8c4d5952b7ae9e44eaf322d064831e1c
Correct spacing before Notifications impact

We should have two new lines between top level headings, and one new
line between the sub-heading.

Corrected the space before Notifications impact to be a single line.

Change-Id: I17cfda30eacb8035de4d2b8ff22de819cfd4cdec
Merge "Updates to the template about policy changes"
Merge "Make bullet lists more consistent"
Merge "Move usage notes back into body of template"
Merge "Correct spacing before Notifications impact"