Improve document on `fp_skip_bad_box`

### Summary The feature `fp_skip_bad_box` needs a better documentation. ### DP-GEN Version devel ### Platform, Python Version, etc _No response_ ### Details Current the documentation of the `fp_skip_bad_box` basically explains nothing. https://github.com/deepmodeling/dpgen/blob/6e591dcd761e45decdd9d1d213a2a8fc72dc59c8/dpgen/generator/arginfo.py#L653-L655 after digging deeper into the code i find that the option may be provided by a string like "length_ratio:3;height_ratio:3;wrap_ratio:.5;tilt_ratio:.5", the meaning of the keywords and the suggested values are not documented. This issue asks for a detailed documentation on this feature.