doc: Remove a mention to :dedent: 0

`:dedent: 0` option for the code-block directive is a secret feature.
So it should not be mentioned in our docs.

Author: tk0miya

CHANGES

@@ -78,10 +78,8 @@ Bugs fixed
 * #9979: Error level messages were displayed as warning messages
 * #10057: Failed to scan documents if the project is placed onto the root
   directory
-* #9636: code-block: ``:dedent:`` handled numeric argument ``0`` like no argument was given
 * #9636: code-block: ``:dedent:`` without argument did strip newlines
 
-
 Testing
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