docs: Note 'config' object shouldn't be accessed after initial load

Update Code_Overview.md to note that the config object should not be
stored after the "config loading phase".

Remove a few inadvertent cases where a 'config' object was stored
in module member variables.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/screws_tilt_adjust.py

@@ -9,7 +9,6 @@ from . import probe
 
 class ScrewsTiltAdjust:
     def __init__(self, config):
-        self.config = config
         self.printer = config.get_printer()
         self.screws = []
         self.results = {}
@@ -33,7 +32,7 @@ class ScrewsTiltAdjust:
                                        default='CW-M3')
         # Initialize ProbePointsHelper
         points = [coord for coord, name in self.screws]
-        self.probe_helper = probe.ProbePointsHelper(self.config,
+        self.probe_helper = probe.ProbePointsHelper(config,
                                                     self.probe_finalize,
                                                     default_points=points)
         self.probe_helper.minimum_points(3)