Auto-disable racket-xp-mode when racket-pdb-mode enabled

Write some pros and cons in the doc string for racket-pdb-mode.

racket-pdb.el

@@ -27,6 +27,7 @@
 (require 'xref)
 
 (declare-function racket-complete-at-point "racket-edit" ())
+(declare-function racket-xp-mode "racket-xp" (&optional arg))
 
 (defvar racket-pdb-control-c-hash-keymap
   (racket--easy-keymap-define
@@ -58,8 +59,45 @@
 (define-minor-mode racket-pdb-mode
   "Like `racket-xp-mode' but using the pdb package, which must be installed.
 
-For now, please refer to the documentation for `racket-xp-mode',
-most of which is relevant to this mode."
+There are several advantages of `racket-pdb-mode':
+
+- The `racket-pdb-rename' command works across mulitple (known,
+  analyzed) files, and is smarter about what should be named than
+  a grep (text search) approach. Similarly `xref-find-references'
+  yields better, non-grep results.
+
+- The pdb database is queried only as needed to present things on
+  screen or to run commands like `xref-find-definition'.
+  Performance is much better for larger files. Although the
+  underlying drracket/check-syntax analysis can still take a long
+  time (e.g. ~10 seconds for something like
+  private/class-internal.rkt), Emacs is never blocked by
+  re-propertizing the entire buffer. Also, if the file hasn't
+  changed since being last visited and analyzed, the previous
+  results can be reused, even if the buffer has been closed in
+  Emacs or Emacs has been restarted.
+
+The disadvantages:
+
+- The pdb Racket package must be installed.
+
+- The pdb Racket package requires very new versions of the
+  drracket-tool-text-lib package as wellas Racket itself. The
+  newer versions improve the ability to analyze code especially
+  to understand the graph of sites that must all be renamed
+  together.
+
+- The pdb \"database\" uses some disk space, on the order of 100
+  MB for an analysis of about 8,000 files in the Racket main
+  distribution. On the other hand you can limit the analysis to
+  projects you're actively working on, if that is the extent of
+  the scope for which you expect renaming to work. In other
+  words, you won't need such a large database unless you want
+  to rename something like \"define\" in racket/base. :)
+
+Until this doc string is further rewritten, please refer to the
+documentation for `racket-xp-mode', most of which is relevant to
+this mode."
   :lighter racket-pdb-mode-lighter
   :keymap racket-pdb-mode-map
   (unless (eq major-mode 'racket-mode)
@@ -68,6 +106,10 @@ most of which is relevant to this mode."
   (unless (racket--cmd/await nil '(pdb-available?))
     (setq racket-pdb-mode nil)
     (user-error "The `pdb` package is not available so racket-pdb-mode cannot be used; use racket-xp-mode instead"))
+  ;; Enabling both `racket-pdb-mode' and `racket-xp-mode' isn't
+  ;; supported so automatically disable latter.
+  (when (bound-and-true-p racket-xp-mode)
+    (racket-xp-mode -1))
   (cond
    (racket-pdb-mode
     (add-hook 'after-change-functions