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

Configurations/unix-Makefile.tmpl

@@ -708,9 +708,11 @@ update: generate errors ordinals
 generate: generate_apps generate_crypto_bn generate_crypto_objects \
           generate_crypto_conf generate_crypto_asn1 generate_fuzz_oids
 
+.PHONY: doc-nits
 doc-nits:
 	(cd $(SRCDIR); $(PERL) util/find-doc-nits -n -p ) >doc-nits
-	if [ -s doc-nits ] ; then cat doc-nits; rm doc-nits ; exit 1; fi
+	@if [ -s doc-nits ] ; then cat doc-nits ; exit 1; \
+	else echo 'doc-nits: no errors.'; rm doc-nits ; fi
 
 # Test coverage is a good idea for the future
 #coverage: $(PROGRAMS) $(TESTPROGRAMS)