Add "How to Validate Inequalities" documentation.

[shawnbrown]

Add how-to documentation for inequalities.

Should demonstrate:

• using validate.interval() to create left- and right-bounded intervals (for greater-than-or-equal-to and less-than-or-equal-to)
• using functions for implementing greater-than or less than (but not equal-to)
• and maybe interval disjunction (e.g. a combined x < min or max < x)