Remove an obsolete comment about srp_scores/3
srp/3, srp_scores/3: Expose labeling Options

Previously, only K from `assumptions(K)` option was exposed.
Now the user can esp. control the labeling procedure.
srp_scores/3: Add comments about clarifying constraint model
Documentation: Add reference to srp/3 from srp/2
Cosmetic: Fix capitalization in srp/2 comment
Add description to srp_scores/3
random_preferences/2: Add example of usage
Cosmetic: random.pl: Fix setrand code example