Update documentation for format strings

[Legoclones]

By default, when creating a FmtStr class or using fmtstr_payload(), all the documentation (and examples) state that the value in the writes dictionary should be an int. However, this int is automatically expanded to a bytestring of size long internally. If someone wants to write data shorter than a long, they need to provide a bytestring as the value instead of an integer (either raw using b'\x37\x13' or helper functions like p16(0x1337)). However, this is never documented anywhere or provided in any examples.

This has led to a number of issues being filed, such as:

• fmtstr_payload write_size argument only works for multiple bytes #2500
• pwnlib.fmtstr - impossible overlapping byte writes #2499
• FmtStr - Write only N byte instead of 8 #1698

This pull requests expands on the current documentation to explicitly lay out whether an int or bytes argument is needed and examples are provided to show how providing a bytestring creates a different format string payload.

Hopefully this helps to reduce confusion and prevent similar future issues from being filed.

[peace-maker]

Thank you! ❤️