Enforce typing style for RequestHeader docs

[nickvergessen]

The suggestion is to enforce a certain casing for the headers for consistency.
Suggestion:

• only lowercase and -
• only lowercase and _
• only uppercase and _

IRequest::getHeader() unifies it to that anyway: https://github.com/nextcloud/server/blob/9836e9b16484582d309c8437ab46d82e34956941/lib/private/AppFramework/Http/Request.php#L262
So it's just a matter of clear / aligned documentation.