fix(vh): document how to disable crossplay (#4969)

* fix(vh): add crossplay setting to control -crossplay

-crossplay was hard-coded in vhserver's startparameters, so Steam-only
servers could not turn off the PlayFab relay without overriding the
whole startparameters line. Add crossplay="true" (default unchanged)
and only pass -crossplay when it is "true".

Fixes #4930

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(vh): document how to disable crossplay instead of adding a toggle

Drop the crossplay setting and the conditional in startparameters.
Keep -crossplay in the defaults, consistent with arkserver, and explain
how Steam-only servers remove it by overriding startparameters.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

lgsm/config-default/config-lgsm/vhserver/_default.cfg

@@ -50,6 +50,9 @@ instanceid="1"
 worldmodifiers=""
 
 ## Server Parameters | https://docs.linuxgsm.com/configuration/start-parameters#additional-parameters
+# -crossplay routes connections through the PlayFab relay so Xbox/Game Pass players can join.
+# Steam-only servers joined by direct IP: copy startparameters to your instance config and remove -crossplay.
+# https://docs.linuxgsm.com/game-servers/valheim#crossplay
 startparameters="-name '${servername}' -password ${serverpassword} -port ${port} -world ${worldname} -public ${public} -savedir '${savedir}' -saveinterval ${saveinterval} -backups ${backups} -backupshort ${backupshort} -backuplong ${backuplong} -crossplay -instanceid ${instanceid} ${logFile:+ -logFile '${logFile}'} ${worldmodifiers:+ ${worldmodifiers}}"
 
 #### LinuxGSM Settings ####