docs(MessageEmbed): document the constructor

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

This PR documents the constructor of MessageEmbed, which somehow was not yet documented.
Resolves #4000?

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[NotSugden]

you previously agreed with this review comment to remove the docstring (on #3894)

[SpaceEEC]

It was since eslint would complain about the "missing"paramter (or rather private pseudo-"overload").

This way eslint is fine, so I think we are clear to go about this.