## Here is the script I will enabled in a SystemD Unit for testing purposes
[centos@localhost ~]$ cat ~/test.sh
#!/bin/sh
echo "hello"

## Here is the documentation file I will reference in the SystemD Unit
[centos@localhost ~]$ cat ~/test.txt

this is a piece of test documentation

## Here is the System D Unit file
[centos@localhost ~]$ cat /etc/systemd/system/test1.service

[Unit]
Description=Test
Documentation=file://localhost/home/centos/test.txt

[Service]
ExecStart=/home/centos/test.sh

[Install]
WantedBy=multi-user.target

[centos@localhost ~]$ sudo systemctl daemon-reload
[centos@localhost ~]$ sudo systemctl restart test1.service
[centos@localhost ~]$ sudo systemctl status test1.service
● test1.service - Test
   Loaded: loaded (/etc/systemd/system/test1.service; enabled; vendor preset: disabled)
   Active: inactive (dead) since Sun 2019-02-17 18:37:13 GMT; 6s ago
     Docs: file://localhost/home/centos/test.txt
  Process: 16036 ExecStart=/home/centos/test.sh (code=exited, status=0/SUCCESS)
 Main PID: 16036 (code=exited, status=0/SUCCESS)

Feb 17 18:37:13 localhost.localdomain systemd[1]: Started Test.
Feb 17 18:37:13 localhost.localdomain test.sh[16036]: hello

## Here is the problem:
[centos@localhost ~]$ systemctl help test1.service
Can't show: file://localhost/home/centos/test.txt

file://localhost/home/centos/test.txt
file:///home/centos/test.txt
file:/home/centos/test.txt