Document the multiline option for strings in user configurations

https://redmine.teklia.com/issues/7870
https://redmine.teklia.com/issues/9102

Requires arkindex/backend#1868 (closed) and arkindex/frontend#1511 (closed) (in Arkindex 1.7.1)

The docs on string user configuration parameters should mention that an optional multiline boolean attribute will show a larger text input that allows line breaks. Note that a string with line breaks in a regular input will still be allowed. There is no specific validation.