docs: document size-mismatch behavior of fixed-size conversions

[nlohmann]

Summary

Conversions whose element count is fixed by the destination C++ type rather than by the JSON value read exactly the elements they need via at(i) and never compare that number to the JSON array's size. This affects:

• get<std::pair<A, B>>
• get<std::tuple<Ts...>>
• get<std::array<T, N>>
• the C-array from_json overloads (T[N])
• std::map / std::unordered_map with a non-string key type (read from an array of two-element arrays)

The two mismatch directions behave differently, and neither was documented in conversions.md, get.md, or from_json.md:

• too many elements → the surplus is silently discarded, no exception;
• too few elements → at throws out_of_range.401, i.e. an out-of-range error rather than a type_error, even though the cause is a shape mismatch.

Verified against current develop for all five destination kinds in both directions:

pair excess: 1,2                    pair short:    out_of_range.401
tuple excess: 1,2                   tuple short:   out_of_range.401
array excess: 1,2                   array short:   out_of_range.401
C-array excess: 1,2                 C-array short: out_of_range.401
map<int,int> excess: {1: 2}         map short:     out_of_range.401

Correction to an existing warning

conversions.md already carried a warning, but it covered only std::array and claimed that a too-short JSON array leaves the remaining elements default-constructed with no exception thrown. That is not what happens — from_json_array_impl for std::array uses j.at(i), so it throws out_of_range.401 like the others. This PR generalizes the warning to every fixed-size destination and corrects the shortfall direction.

Non-goals

This documents the current behavior only. Making these conversions strict about size would break working code and belongs in the 4.0 bucket.

Breaking changes to the public API

None — documentation only, no code or headers touched.

---

This pull request was prepared by Claude Code.