Update Sophia syntax docs to include missing information about existing syntax (#405)

* Add main contract, contract interface, and guards to the docs syntax

* Use Sep1 instead of Sep for the GuardedDef

* Add guarded case for switches

* Change '=' to '::=' in GuardedDef

* Add Using

* Add '|>' binary operator to aeso_syntax

* Add assign patter

* Fix typos

* Add polymorphism implmented interface syntax

src/aeso_syntax.erl

@@ -106,7 +106,7 @@
 
 -type bin_op() :: '+' | '-' | '*' | '/' | mod | '^'
                 | '++' | '::' | '<' | '>' | '=<' | '>=' | '==' | '!='
-                | '||' | '&&' | '..'.
+                | '||' | '&&' | '..' | '|>'.
 -type un_op() :: '-' | '!'.
 
 -type expr()