Update docs to reflect that you can join calls without the labs flags

[robintown]

Docs update for element-hq/matrix-react-sdk#95

[RiotRobot]

The backport to staging failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-staging staging
# Navigate to the new working tree
cd .worktrees/backport-staging
# Create a new branch
git switch --create backport-28113-to-staging
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 e428a1eb536bce454b78fb02e55f061762dd7d2d
# Push it to GitHub
git push --set-upstream origin backport-28113-to-staging
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-staging

Then, create a pull request where the base branch is staging and the compare/head branch is backport-28113-to-staging.