Enrich the details of `sim_fixed_n`

[LittleBeannie]

As aligned in our meeting on February 6, we are closing issue #256 .

Decision Rationale: The sim_fixed_n() and sim_gs_n() functions were developed as high-level wrappers to simplify the most common use cases for users. For users who wish to explore more advanced scenarios, such as:

• Tests other than the log-rank or weighted log-rank (Fleming-Harrington).
• Timing options beyond the five provided defaults.
  we recommend using the more flexible, lower-level sim_pw_surv() function. A detailed guide for this is available at https://merck.github.io/simtrial/articles/sim_fixed_design_custom.html.

Next Steps: We will update the @details for sim_fixed_n() to incorporate the above info.