Documentation missing default language_in and language_out modes

In wiki page https://github.com/google/closure-compiler/wiki/Flags-and-Options, the docs

```
--language_in VAL
Sets the language spec to which input sources should conform. Options: ECMASCRIPT3, ECMASCRIPT5, ECMASCRIPT5_STRICT, ECMASCRIPT6_TYPED (experimental), ECMASCRIPT_2015, ECMASCRIPT_2016, ECMASCRIPT_2017, ECMASCRIPT_2018, ECMASCRIPT_2019, STABLE, ECMASCRIPT_NEXT

--language_out VAL
Sets the language spec to which output should conform. Options: ECMASCRIPT3, ECMASCRIPT5, ECMASCRIPT5_STRICT, ECMASCRIPT_2015, ECMASCRIPT_2016, ECMASCRIPT_2017, ECMASCRIPT_2018, ECMASCRIPT_2019, STABLE
```
do not say what are the default options for these settings. That would be good to add?

Also, it would be good to describe the behavior of what happens if one passes only one of `--language_in`, or `--language_out`, but not the other? (e.g. does `--language_in ECMASCRIPT_2018` without an explicitly specified `--language_out` directive imply `--language_out ECMASCRIPT_2018`? Or does e.g. `--language_out ECMASCRIPT_2018` without an explicitly specified `--language_in` directive imply `--language_in ECMASCRIPT_2018`?)