Change PlugClean description (#853)

Be more clear about uninstalling plugin directories.

README.md

@@ -148,7 +148,7 @@ Reload .vimrc and `:PlugInstall` to install plugins.
 | ----------------------------------- | ------------------------------------------------------------------ |
 | `PlugInstall [name ...] [#threads]` | Install plugins                                                    |
 | `PlugUpdate [name ...] [#threads]`  | Install or update plugins                                          |
-| `PlugClean[!]`                      | Remove unused directories (bang version will clean without prompt) |
+| `PlugClean[!]`                      | Remove unlisted plugins (bang version will clean without prompt) |
 | `PlugUpgrade`                       | Upgrade vim-plug itself                                            |
 | `PlugStatus`                        | Check the status of plugins                                        |
 | `PlugDiff`                          | Examine changes from the previous update and the pending changes   |

doc/plug.txt

@@ -194,7 +194,7 @@ Reload .vimrc and `:PlugInstall` to install plugins.
  ------------------------------------+-------------------------------------------------------------------
   `PlugInstall [name ...] [#threads]`  | Install plugins
   `PlugUpdate [name ...] [#threads]`   | Install or update plugins
-  `PlugClean[!]`                       | Remove unused directories (bang version will clean without prompt)
+  `PlugClean[!]`                       | Remove unlisted plugins (bang version will clean without prompt)
   `PlugUpgrade`                        | Upgrade vim-plug itself
   `PlugStatus`                         | Check the status of plugins
   `PlugDiff`                           | Examine changes from the previous update and the pending changes