Updated docs for setting memory
Updated docs for setting memory

- switched from scientific notation to human-readable
- added section for `unique-kmers.py` and `-U` option (closes #1558)
- updated diginorm FPR threshold from 10% to 20%

ged-lab/khmer,souravsingh/khmer,souravsingh/khmer,ged-lab/khmer,ged-lab/khmer,souravsingh/khmer