Describe program analysis for widening bounds for nt_array_ptr

[dtarditi]

We need to describe the program analysis for widening bounds for nt_array_ptr. We also need to describe how it hooks into checking of bounds declarations and insertion of bounds checks:

• For checking of bounds declarations, the widened bounds is always used.
• For bounds checks, the widened bounds are used only for expressions whose bounds are derived from a variable with inferred bounds.