Support security in desc block

[fotos]

Adds support for security in desc block.

Proposing the change since options hash parameters are marked as deprecated and the bug in ruby-grape/grape-swagger#706 prevents using both (options are not merged into the block).

desc `Create User` do
  security [
    # security schemes
  ]
  # …
end

Includes some minor formatting changes in the spec (in separate commits) and I didn't update the README.md since it already points to grape-swagger for more info.

Related to #1791 and (slightly) to ruby-grape/grape-swagger#729.

[dblock]

Looks good, merged.