Clarify move*() docs (#888)

Fixes #830

Author: RyanZim

docs/move-sync.md

@@ -3,7 +3,7 @@
 Moves a file or directory, even across devices.
 
 - `src` `<String>`
-- `dest` `<String>`
+- `dest` `<String>` Note: When `src` is a file, `dest` must be a file and when `src` is a directory, `dest` must be a directory.
 - `options` `<Object>`
   - `overwrite` `<boolean>`: overwrite existing file or directory, default is `false`.
 

docs/move.md

@@ -3,7 +3,7 @@
 Moves a file or directory, even across devices.
 
 - `src` `<String>`
-- `dest` `<String>`
+- `dest` `<String>` Note: When `src` is a file, `dest` must be a file and when `src` is a directory, `dest` must be a directory.
 - `options` `<Object>`
   - `overwrite` `<boolean>`: overwrite existing file or directory, default is `false`.
 - `callback` `<Function>`