docs/maintainer-guide.txt: add "make -j 8 distcheck-completeness" into the mix

Signed-off-by: Jim Klimov <jimklimov+nut@gmail.com>

Author: jimklimov

docs/maintainer-guide.txt

@@ -259,7 +259,8 @@ VERSION_FORCED_SEMVER:NUT_VERSION_FORCED_SEMVER='2.8.3'
       --enable-warnings --enable-Werror --enable-maintainer-mode && \
    make -j 8 all && \
    make -j 8 spellcheck && \
-   make -j 8 distcheck
+   make -j 8 distcheck && \
+   make -j 8 distcheck-completeness
 ----
 * create an annotated GPG-signed tag v<incremented version> (ex: `v2.8.0-rc1`):
 +