Define what it means to satisfy a security scheme

[mmccool]

Resolves #1667

Add following informative text to the "Security Scheme" section to define what it means to "satisfy" a security scheme. This intentionally does not talk about details (e.g. credentials, protocols, etc), as these will be defined by each scheme.

Each security scheme object used in a TD defines a set of requirements to be met before access can be granted. We say a security scheme is satisfied when all its requirements are met. In some cases requirements from multiple security schemes will have to be met before access can be granted.

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[sebastiankb]

from today's TD call, decided to merge.
@mmccool will resolve the conflict and will merge this PR then