Clarify hash documentation after review

- Say "may hash differently" for null, false, and numbers, since a
  collision across types is possible.
- Name the storage types (signed integer, unsigned integer,
  floating-point number) instead of example literals.
- Explain that the hash survives converting an integer to
  number_float_t but not the lossy conversion back, and that unequal
  numbers may share a hash.
- State that the example hash values are illustrative only and vary by
  platform, compiler, compiler version, and library version.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -7636,7 +7636,7 @@ std::size_t hash_iteratively(const BasicJsonType& j);
 @brief hash a JSON value
 
 The hash function tries to rely on std::hash where possible. Furthermore, the
-type of the JSON value is taken into account, so null, false, and numbers all
+type of the JSON value is taken into account, so null, false, and numbers may
 hash differently from each other, but any two numbers that compare equal
 under operator== hash equally regardless of which of number_integer,
 number_unsigned, or number_float actually holds the value.
@@ -7724,6 +7724,10 @@ std::size_t hash(const BasicJsonType& j, const std::size_t depth = 0)
             // the same number_float_t, so all numbers share one type tag and
             // hash that converted value. Adding zero turns -0.0 (equal to 0)
             // into 0.0, as std::hash need not map both to the same hash.
+            // The converse does not hold: converting a number_float_t value
+            // to an integer type is lossy, so the result can hash
+            // differently, and unequal numbers that convert to the same
+            // number_float_t (e.g., 2^53 and 2^53 + 1) share a hash.
             const auto number_type = static_cast<std::size_t>(BasicJsonType::value_t::number_float);
             const auto value = j.template get<number_float_t>() + static_cast<number_float_t>(0);
             const auto h = std::hash<number_float_t> {}(value);