Remove/modify misleading togglePaneZoom documentation

[DHowett]

I recently started playing with Panes in Terminal and immediately wanted the Pane Focus Zoom feature and went looking for it. I see it was added but then turned off in stable due to problems leaving a blank screen as noted in #7252 and #7630. The documentation directly on the panes page itself (https://docs.microsoft.com/en-us/windows/terminal/panes) doesn't talk about it which is good but the documentation on the Custom Actions page (https://docs.microsoft.com/en-us/windows/terminal/customize-settings/actions#zoom-a-pane) still show it as an option. I tried using this getting very confused why it wasn't working until I found the PR previously mentioned.

TL;DR togglePaneZoom is currently disabled in Stable but documentation here seems to say otherwise.

Requested Fix: Remove documentation on this page OR (personally preferred) Add a note next to the command listing saying it's only currently available in Preview builds

From @jjspace in microsoft/terminal#7812

[DHowett]

/cc @cinnamon-msft something we accidentally promoted from preview to stable even though we didn't promote the feature.

[jjspace]

Woops, I didn't realize there was a separate repo for the docs, thanks for moving this over @DHowett!

[mattwojo]

This feature has now made it to the stable release so closing this issue. :)