Auto merge of #2320 - ota42y:ota42y/alis_command_doc, r=hsbt

add install alias documentation

# Description:
Until now the `i` command work but it was unintended.
[this change](#2308) made it an official command.
So we had better write an explanation in help
______________

# Tasks:

- [x] Describe the problem / feature
- [-] Write tests
- [x] Write code to solve the problem
- [ ] Get code review from coworkers / friends

I will abide by the [code of conduct](https://github.com/rubygems/rubygems/blob/master/CODE_OF_CONDUCT.md).

lib/rubygems/commands/install_command.rb

@@ -117,6 +117,13 @@ def description # :nodoc:
   some_extension_gem (1.0)
   $
 
+Command Alias
+==========================
+
+You can use `i` command instead of `install`.
+
+  $ gem i GEMNAME
+
     EOF
   end