Update MIME docstring

[nathanrboyer]

Closes #56768

[stevengj]

Suggested change

	A parametric type representing a standard internet data format.
	A parametric type representing a content type/format, which is used to define [`show`](@ref)
	methods to output objects in different formats and allows [`display(x)`](@ref) to detect
	which formats are available for a given `x` in a particular display environment.

[nathanrboyer]

The MIME type is only used for the show function? Should the docstring limit the type's usage in this way?

[stevengj]

Suggested change

	[`@MIME_str`](@ref) is defined to simplify creation of singleton types in this way,
	e.g. `MIME"text/plain"`.
	The string macro [`@MIME_str`](@ref) allow you to specify `MIME{Symbol("...")}` types more succinctly
	as `MIME"..."`, for example `MIME"text/plain"`.

[nathanrboyer]

I slightly rewrote this.

[stevengj]

Suggested change

	A `MIME` object can be passed as the second argument to [`show`](@ref)
	to request output in that format.
	A `mime::MIME` instance can be passed as the second argument to [`show(io, mime, x)`](@ref)
	to request output in that format (if it is implemented for `x`), as well as to various other functions like
	[`showable`](@ref), [`repr`](@ref), and [`display`](@ref); most such functions also allow you to
	pass a string (e.g. `"text/plain"`) that will be converted to a `MIME` instance for you.

[nathanrboyer]

I slightly rewrote this to avoid "you".