Update description for max_backtrace_frames

The previous wording for error_collector.max_backtrace_frames
could be interpreted that the beginning and end would be removed
and the middle would be preserved. This update makes the
code's behavior more clear.

lib/new_relic/agent/configuration/default_source.rb

@@ -720,7 +720,7 @@ def self.enforce_fallback(allowed_values: nil, fallback: nil)
           :public => true,
           :type => Integer,
           :allowed_from_server => false,
-          :description => 'Defines the maximum number of frames in an error backtrace. Backtraces over this amount are truncated at the beginning and end.'
+          :description => 'Defines the maximum number of frames in an error backtrace. Backtraces over this amount are truncated in the middle, preserving the beginning and the end of the stack trace.'
         },
         :'error_collector.max_event_samples_stored' => {
           :default => 100,