docs(macros): document that bind parameter nullability is not compile-checked (#4345)

Restructures the "Nullability: Bind Parameters" section of the query!
macro documentation to state upfront that the nullability of bind
parameters is not verified at compile time, why (SQLx does not parse
SQL), and give the concrete example from #2642.

The previous phrasing understated the limitation and hid it behind
guidance about WHERE clauses, which is now demoted to a sub-section.

Closes #2642