(docs/types): add comments to some remaining declarations

- clarify why some of these internal declarations/re-declarations
  are necessary
  - I tried removing them because I didn't know why they were needed,
    so this documentation should be helpful

src/env.d.ts

@@ -1,7 +1,7 @@
-declare module 'asyncro';
-declare module 'enquirer';
-declare module 'jpjs';
-declare module 'tiny-glob/sync';
+declare module 'asyncro'; // doesn't have types (unmerged 2+ year old PR: https://github.com/developit/asyncro/pull/10)
+declare module 'enquirer'; // doesn't have types for Input or Select
+declare module 'jpjs'; // doesn't ship types (written in TS though)
+declare module 'tiny-glob/sync'; // /sync isn't typed (but maybe we can use async?)
 
 // Patch Babel
 // @see line 226 of https://unpkg.com/@babel/core@7.4.4/lib/index.js