Improve the output of `make doc-nits`

- Print positive feedback in the case when 'make doc-nits' finds no errors.
- Other than before, keep the 'doc-nits' output file only in case of errors
  and remove it if it is empty.
- Declare 'doc-nits' as a phony make target to facilitate rerunning
  'make doc-nits' without having to remove the output file first.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6517)