lint-collection-docs: improve output formatting

[felixfontein]

Example output:

/path/to/ansible_collections/ansible/utils$ antsibull-docs lint-collection-docs . --plugin-docs
plugins/test/in_any_network.py:0:0: Did not return correct DOCUMENTATION
plugins/test/in_any_network.py:0:0: 2 validation errors for PositionalDocSchema
                                    doc -> options -> ip -> description -> 1
                                      str type expected (type=type_error.str)
                                    doc -> options -> networks -> description -> 1
                                      str type expected (type=type_error.str)
plugins/test/in_network.py:0:0: Did not return correct DOCUMENTATION
plugins/test/in_network.py:0:0: 2 validation errors for PositionalDocSchema
                                doc -> options -> ip -> description -> 1
                                  str type expected (type=type_error.str)
                                doc -> options -> network -> description -> 1
                                  str type expected (type=type_error.str)
plugins/test/in_one_network.py:0:0: Did not return correct DOCUMENTATION
plugins/test/in_one_network.py:0:0: 2 validation errors for PositionalDocSchema
                                    doc -> options -> ip -> description -> 1
                                      str type expected (type=type_error.str)
                                    doc -> options -> networks -> description -> 1
                                      str type expected (type=type_error.str)
[...]

[briantist]

lgtm!