[shell] 'kill' completion will now require trigger sequence (**)

'kill **<tab>' instead of 'kill <tab>' just like any other completions.

Close #2716
Close #385

README.md

@@ -382,12 +382,11 @@ cd ~/github/fzf**<TAB>
 
 #### Process IDs
 
-Fuzzy completion for PIDs is provided for kill command. In this case,
-there is no trigger sequence; just press the tab key after the kill command.
+Fuzzy completion for PIDs is provided for kill command.
 
 ```sh
 # Can select multiple processes with <TAB> or <Shift-TAB> keys
-kill -9 <TAB>
+kill -9 **<TAB>
 ```
 
 #### Host names