Improve introduction and getters usability

When someone first joins a room, they see some commands (`mailbox`,
`owner`, ..) and they know they are getters and setters, but they have
no good example as to how to use them.

Is it `!pm mailbox SOMETHING` or `!pm mailbox=SOMETHING` or something
else?

It's better if the introduction text gives you the full command you need
to get started (e.g. `!pm mailbox SOME_MAILBOX`), instead of a partial
command that you don't know how to use (e.g. `!pm mailbox` - this is
merely a getter and will not set your mailbox to `SOME_MAILBOX`).

Starting from this, I thought it would be a good idea to make all
option getters tell you how the commands are to be used. If you send
`!pm mailbox` and it tells you "not yet set", it should also tell you
how to actually set it (e.g. `!pm mailbox VALUE`).

bot/command.go

@@ -181,7 +181,7 @@ func (b *Bot) sendIntroduction(ctx context.Context, roomID id.RoomID) {
 	msg.WriteString(b.prefix)
 	msg.WriteString(" ")
 	msg.WriteString(roomOptionMailbox)
-	msg.WriteString("` command.\n")
+	msg.WriteString(" SOME_INBOX` command.\n")
 
 	msg.WriteString("You will then be able to send emails to `SOME_INBOX@")
 	msg.WriteString(b.domain)

bot/command_owner.go

@@ -3,6 +3,7 @@ package bot
 import (
 	"context"
 	"fmt"
+	"strings"
 )
 
 func (b *Bot) runStop(ctx context.Context) {
@@ -48,7 +49,15 @@ func (b *Bot) getOption(ctx context.Context, name string) {
 
 	value := cfg.Get(name)
 	if value == "" {
-		b.SendNotice(ctx, evt.RoomID, fmt.Sprintf("`%s` is not set, kupo.", name))
+		var msg strings.Builder
+		msg.WriteString(fmt.Sprintf("`%s` is not set, kupo.", name))
+		msg.WriteString("\n")
+		msg.WriteString(fmt.Sprintf(
+			"To set it, send a `%s %s VALUE` command.",
+			b.prefix,
+			name,
+		))
+		b.SendNotice(ctx, evt.RoomID, msg.String())
 		return
 	}
 
@@ -56,7 +65,15 @@ func (b *Bot) getOption(ctx context.Context, name string) {
 		value = value + "@" + b.domain
 	}
 
-	b.SendNotice(ctx, evt.RoomID, fmt.Sprintf("`%s` of this room is `%s`", name, value))
+	var msg strings.Builder
+	msg.WriteString(fmt.Sprintf("`%s` of this room is `%s`", name, value))
+	msg.WriteString("\n")
+	msg.WriteString(fmt.Sprintf(
+		"To set it to a new value, send a `%s %s VALUE` command.",
+		b.prefix,
+		name,
+	))
+	b.SendNotice(ctx, evt.RoomID, msg.String())
 }
 
 func (b *Bot) setOption(ctx context.Context, name, value string) {