Clarify documentation for `--all` option in `cache clear` command

Resolves: #1162

poetry/console/commands/cache/clear.py

@@ -9,7 +9,7 @@ class CacheClearCommand(Command):
 
 
     cache:clear
     cache:clear
         { cache : The name of the cache to clear. }
         { cache : The name of the cache to clear. }
-        { --all : Clear all caches. }
+        { --all : Clear all entries in cache. }
     """
     """
 
 
     def handle(self):
     def handle(self):