[move-book] Added block doc comment option

Move seems to support block doc comments /** */.

It seems that the document does not make style recommendations on the
comment style, therefore listing all the possible comment options here.

Closes: diem#125

language/documentation/book/src/coding-conventions.md

@@ -65,7 +65,7 @@ module 0x1::Importer {
 ## Comments
 
 - Each module, struct, and public function declaration should be commented
-- Move has both doc comments `///`, regular single-line comments `//`, and block comments `/* */`
+- Move has doc comments `///`, regular single-line comments `//`, block comments `/* */`, and block doc comments `/** */`
 
 ## Formatting