man: document paths under /usr/local in standard-conf.xml

man: document paths under /usr/local in standard-conf.xml

standard-conf.xml is currently included by:
man/binfmt.d.xml
man/environment.d.xml
man/modules-load.d.xml
man/sysctl.d.xml
man/coredump.conf.xml
man/journal-remote.conf.xml
man/journal-upload.conf.xml
man/journald.conf.xml
man/logind.conf.xml
man/networkd.conf.xml
man/resolved.conf.xml
man/systemd-sleep.conf.xml
man/systemd-system.conf.xml

All those programs actually use CONF_PATHS_NULSTR or CONF_PATHS_STRV,
so this changes the documentation to match code.