docs: clarify server.on() and server.emit() are aliases for the main namespace

[tech00exploere]

Description

The Server API docs showed examples of io.on() and io.emit() but never explained
that these are aliases for the main namespace (/), which was confusing for new users.

Other methods like server.use(), server.fetchSockets(), and server.disconnectSockets()
already had "Alias for io.of("/")..." notes — this PR makes server.on() and
server.emit() consistent with that pattern.

Changes

• Added alias note to server.on() clarifying it maps to io.of("/").on()
• Added alias note to server.emit() clarifying it maps to io.of("/").emit()

Related

Fixes socketio/socket.io#4608

[vercel]

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