(Docs) Update find_file() comment in files.rb

Testing confirms that the `find_file()` function may indeed be used to
verify the existence of a file specified by absolute path, with the
caveat that this check appears to be performed prior to any resource
changes made by the same code.

The following code, when run via "puppet apply", accurately reports the
existence or non-existence of a test file *before* deleting that file.

```
$testpath = '/tmp/testfile'
file { $testpath: ensure => absent }
if find_file($testpath) {
  notify { "$testpath exists.": }
} else {
  notify { "$testpath does not exist.": }
}
```

lib/puppet/parser/files.rb

@@ -29,9 +29,10 @@ def find_manifests_in_modules(pattern, environment)
   #   * modulename/filename selector: a file is found in the file directory
   #     of the named module.
   #
-  # In the second case a nil is returned if there isn't a file found. In the
-  # first case (absolute path), there is no existence check done and so the
-  # path will be returned even if there isn't a file available.
+  # If this function is called via "puppet agent", the check for file existence
+  # is performed on the Puppet Primary server. If run via "puppet apply", the
+  # check is performed on the local host. In both cases, the check is performed
+  # before any resources are changed, and a nil is returned if no file is found.
   #
   # @param template [String] the file selector
   # @param environment [Puppet::Node::Environment] the environment in which to search