Ensure YAML-parsable OpenAPI docs

[Geogouz]

As pointed in #563 (review), the OpenAPI docs contained in our web classes should be YAML parsable without causing issues. Specifically about the pointed issue, a simple approach that I follow is to quote my one-line description fields like so:

description: "The scope and the name of the DID (e.g., 'scope:name')."

or for multilines we could be using:

description: |
  Set metadata for a data identifier (DID). If a piece of metadata for a given key
  already exists, it will be handled according to the underlying metadata plugin
  in use. blablabla.

Regarding this, we should maybe mention this somewhere, but we should ideally also introduce somewhere a check about this (either at rucio/documentation side or rucio/rucio).