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.

[stevengj]

Suggested change

	There are over one thousand official media types,
	but in practice only a few are usually supported.
	There are over one thousand official media types, along with innumerable unofficial application-specific
	types (though in practice only a few are usually supported in any given `display` environment).

[stevengj]

Suggested change

Unofficial custom MIMEs are also supported.

It's not clear what is meant by "supported" here.

[stevengj]

Suggested change

	Some common MIMEs are `"text/plain"``, `"text/html"``, `"image/jpeg"``, `"video/mpeg"``.
	Examples of common media types include `"text/plain"``, `"text/html"``, `"image/jpeg"``, `"video/mpeg"``.

[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"`.

[stevengj]

Suggested change

	A `MIME` **object** is created by calling the MIME constructor, either directly,
	A `MIME` **instance** is created by calling the MIME constructor, either directly,

[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.

[stevengj]

Suggested change

	julia> import Base.show
	julia> show(io::IO, ::MIME"text/plain", x::MyType) = print(io, "My Value is ", x.val);
	julia> Base.show(io::IO, ::MIME"text/plain", x::MyType) = print(io, "My Value is ", x.val);