Document what "specialized" means. (#320)

* Document what "specialized" means.

Add a paragraph documenting specialized types and their relationship to
despecialized types.

This is intended to help with #319.

* Update design/mvp/Explainer.md

Co-authored-by: Lann <[email protected]>

* Mention that Canonical ABI has custom reprs for string and flags.

* Update design/mvp/Explainer.md

Co-authored-by: Luke Wagner <[email protected]>

* A few wording adjustments.

* Split a sentance with a semicolon into two sentences.

This makes the sentence structure more consistent.

---------

Co-authored-by: Lann <[email protected]>
Co-authored-by: Luke Wagner <[email protected]>

Author: 3 people

design/mvp/Explainer.md

@@ -647,6 +647,20 @@ defined by the following mapping:
 (result <valtype>? (error <valtype>)?) ↦ (variant (case "ok" <valtype>?) (case "error" <valtype>?))
                                 string ↦ (list char)
 ```
+
+Specialized value types have the same set of semantic values as their
+corresponding despecialized types, but have distinct type constructors
+(which are not type-equal to the unspecialized type constructors) and
+thus have distinct binary encodings. This allows specialized value types to
+convey a more specific intent. For example, `result` isn't just a variant,
+it's a variant that *means* success or failure, so source-code bindings
+can expose it via idiomatic source-language error reporting. Additionally,
+this can sometimes allow values to be represented differently. For example,
+`string` in the Canonical ABI uses various Unicode encodings while
+`list<char>` uses a sequence of 4-byte `char` code points.  Similarly,
+`flags` in the Canonical ABI uses a bit-vector while an equivalent record
+of boolean fields uses a sequence of boolean-valued bytes.
+
 Note that, at least initially, variants are required to have a non-empty list of
 cases. This could be relaxed in the future to allow an empty list of cases, with
 the empty `(variant)` effectively serving as a [empty type] and indicating