elaborate that npm help uses browser

As I mentioned in issue #2501 this is mildly annoying to me, and
probably a few other people. I have added "(in a browser)" to the npm
usage string at the npm help line.

PR-URL: #2502
Credit: @ariccio
Close: #2502
Reviewed-by: @isaacs

lib/utils/npm-usage.js

@@ -16,8 +16,8 @@ npm test           run this project's tests
 npm run <foo>      run the script named <foo>
 npm <command> -h   quick help on <command>
 npm -l             display usage info for all commands
-npm help <term>    search for help on <term>
-npm help npm       more involved overview
+npm help <term>    search for help on <term> (in a browser)
+npm help npm       more involved overview (in a browser)
 
 All commands:
 ${npm.config.get('long') ? usages() : ('\n    ' + wrap(cmdList))}