Update API docs for secret endpoints

[lamppu]

Here's at least a few things that are a bit unclear:

• Is it by design that the secret value can be an empty string? I’m also wondering what happends when a secret is updated with the secret value being an empty string in the request, will it overwrite the earlier secret or ignore it?
• Should it be possible to update the secret name? Currently the PATCH request body has the name in the request schema, however it looks like it actually can’t be / is not updated. (Also, the current example in the docs implies that it could be updated.)
• It also says in the docs for the PATCH requests: “To delete a value, set it explicitly to null.”. However, sending a null value for e.g. the description field results in an error: Illegal input: Unexpected JSON token at offset 15: Expected string literal but 'null' literal was found at path: $.descriptionUse 'coerceInputValues = true' in 'Json {}' builder to coerce nulls if property has a default value.JSON input: {"description":null

[mnonnenmacher]

The secret name parameter was removed in #1128.