Manual Improvement : Perforce

A customer reported a lack of information in Qt Creator Manual.

The customer was trying to let Qt Creator collaborate with Perforce, on Linux.

He was following the document below, and found it does not work for Linux as it is, because it is premising Windows. (And he could not realize that beforehand because the document does not saying it is premising Windows.)

"Qt Creator Manual - Perforce - Configuring Perforce"

https://doc.qt.io/qtcreator/creator-vcs-perforce.html#configuring-perforce

It describes "p4 set P4CONFIG=p4config.txt" is needed (in Windows). However "export P4CONFIG=[pass to p4config.txt]" is needed In Linux, instead.

This should be added in the document for Linux users.