Update help messages to reflect custom index syntax

[chriskim06]

Here are a list of commands that I think need to have their help messages updated:

• install

Here's a list of maybes since a warning is already printed when someone tries to use custom index syntax (but might be nice to include something in the help message as well):

• uninstall
• upgrade

The examples currently show something along the lines of kubectl krew [command] PLUGIN. Once the multi-index feature is released these will need to be updated to include examples of using custom index syntax. Should the help messages be updated to mention something about the concept of a "default" index? Like:

Examples:
  To install one or more plugins from the default index, run:
    kubectl krew install NAME [NAME...]

  To install one or more plugins from a custom index named "foo", run:
    kubectl krew install foo/NAME [foo/NAME...]

[chriskim06]

/area multi-index