Rename global_configvars.adoc to extended_configvars.adoc

[mmattel]

When writing the rogue envvar code #5164, the exported adoc file was named: global_configvars.adoc.

Post a discussion with @kobergj this file (and possibly other related ones) needs to be renamed:
global_configvars.adoc --> extended_configvars.adoc

This is because we will get a file with real global envvars which then will have the global part in its name.

Also see: owncloud/docs-ocis#335 (Special Scope Environment Variables)

[kobergj]

fixed here: #5272