Improve __replace__ typing and documentation

[provinzkraut]

• Add __replace__ method to stub files, so copy.replace doesn't report a type error
• Document __post_init__ behaviour of __replace__ (see __post_init__ not called from replace #874)

[CharString]

For a better flow of the text, I think this block should be moved after

In addition to in __init__, the __post_init__ hook is also called when:

• Decoding into a struct type (e.g. msgspec.json.decode(..., type=MyStruct))
• Converting into a struct type (e.g. msgspec.convert(..., type=MyStruct))