Normalize attributes format on tags

[Marzal]

I saw too many variants so I propose to keep:

• attribute (required): Explanation
• attribute (optional): Explanation

With these rules:

• Use de code format for attributes as when they are mentioned.
• Use bold for (required)
• Separate the explanation with : after the required/optional

[daveajones]

I think I clobbered you with the Image tag file. Can you resolve the conflict and I'll merge?

[Marzal]

@daveajones done. It was the typo fixed on #723