api-endpoints has inconsistent purpose documentation

[Dimiter Naydenov]

juju api-endpoints command, unlike all the other commands, has a "Purpose" documentation string beginning with an uppercase letter. For consistency of both the "juju help commands" output (http://paste.ubuntu.com/8551037/) and "juju help api-endpoints" header (http://paste.ubuntu.com/8551081/), there should be a unit test to verify each command:
 - has Purpose filled-in and it starts with a lowercase letter;
 - additionally, not all commands have a Doc string defined, but of those who have, it's always either beginning with the command name in lowercase (godoc-style) or like a sentence (uppercase letter first).

It's a trivial issue, but fixing it and adding a test will make the CLI user experience slightly better and avoid such issues in the future.

[Dimiter Naydenov (dimitern)]

Fix proposed as https://github.com/juju/juju/pull/904