docs: Update docs for pretty-by-default behavior

This commit updates the user-facing documentation to reflect that pretty-printing is now the default output format.

- The `--pretty` flag documentation in `command_line.rst` has been replaced with documentation for the new `--no-pretty` flag.
- The `config_file.rst` documentation for the `pretty` option has been updated to state that its default is now `True`.

Fixes #19108.

Author: null-dreams

docs/source/command_line.rst

@@ -910,10 +910,10 @@ in error messages.
 
     See :ref:`error-codes` for more information.
 
-.. option:: --pretty
+.. option:: --no-pretty
 
-    Use visually nicer output in error messages: use soft word wrap,
-    show source code snippets, and show error location markers.
+    Disable pretty error messages and use the old, plain output format.
+    Pretty-printing is enable by default.
 
 .. option:: --no-color-output
 

docs/source/config_file.rst

@@ -896,10 +896,11 @@ These options may only be set in the global section (``[mypy]``).
 .. confval:: pretty
 
     :type: boolean
-    :default: False
+    :default: True
 
     Use visually nicer output in error messages: use soft word wrap,
     show source code snippets, and show error location markers.
+    Pretty printing is enabled by default, but can be disabled.
 
 .. confval:: color_output