fix: correct benchmark data generation in Mersenne31 field benchmarks

[DeVikingMark]

trafficstars

Fixed incorrect loop condition in rand_fp4e() and rand_field_elements() functions that used result.capacity() instead of num parameter, which could generate wrong number of test elements.

Changes:

• Replace result.capacity() with num in loop conditions
• Add proper documentation to benchmark helper functions
• Remove TODO comment after implementation

This ensures benchmark functions generate the expected number of test data elements.