Improve the SLD REST API docs about the retrieval of SLD 1.1.0 styles

[giohappy]

The API docs say that for retrieving a style either the ".sld" or the ".css" extension should be used, and that alternatively the Accept header can be set to specify the requested format.
By the way, this is isn't all the truth:

• if you request an SLD 1.1.0 style by appending the ".sld" extension it's automatically transformed to 1.0.0
• if you add the Accept: application/vnd.ogc.se+xml header it isn't taken into consideration since the ".sld" extension has precedence
• the only solution to obtain the original 1.1.0 style is to do not append any extension (it would be nice to have the ".se" extension)

I suggest to document the three points above inside the swagger docs for styles

[Mike7311]

PR: geoserver#6840