Need clarification on "another server might be running; trying to start server anyway"

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.5/static/server-start.html
Description:

Hello,

I am currently facing an issue on Servers that are running PostgreSQL 9.5
started from systemd, and in the case when the Servers power offs the hard
way, and I restart it, I get an error "another server might be running;
trying to start server anyway"

The Documentation section "17.3.1. Server Start-up Failures" should maybe
mention that error and explain, in which cases postgresql thinks that it is
still running, because neither the port is already taken, or are there
logfiles on the server.

I also tried to remove postmaster.pid before starting postgres, but this
strangely doesn't help either.

Regards,
Daniel Migowski