Update config_file docs (#18103)

Changes:
- Fix `strict_equality` markup. It used to be like this:
<img width="760" alt="Снимок экрана 2024-11-04 в 13 47 35"
src="https://github.com/user-attachments/assets/d3e99a72-5216-4dda-8cca-2e30f6d705dc">
- Add `extra_checks` confval

Author: sobolevn

docs/source/config_file.rst

@@ -717,6 +717,14 @@ section of the command line docs.
 
     Note: This option will override disabled error codes from the disable_error_code option.
 
+.. confval:: extra_checks
+
+   :type: boolean
+   :default: False
+
+   This flag enables additional checks that are technically correct but may be impractical in real code.
+   See :option:`mypy --extra-checks` for more info.
+
 .. confval:: implicit_reexport
 
     :type: boolean
@@ -739,23 +747,23 @@ section of the command line docs.
 
 .. confval:: strict_concatenate
 
-    :type: boolean
-    :default: False
+   :type: boolean
+   :default: False
 
-    Make arguments prepended via ``Concatenate`` be truly positional-only.
+   Make arguments prepended via ``Concatenate`` be truly positional-only.
 
 .. confval:: strict_equality
 
-    :type: boolean
-    :default: False
+   :type: boolean
+   :default: False
 
    Prohibit equality checks, identity checks, and container checks between
    non-overlapping types.
 
 .. confval:: strict
 
-    :type: boolean
-    :default: False
+   :type: boolean
+   :default: False
 
    Enable all optional error checking flags.  You can see the list of
    flags enabled by strict mode in the full :option:`mypy --help`