Fix compiler warnings on develop from the API performance stack (#5858)

These PRs were merged while their CI runs were cancelled, so the warnings
went unnoticed. All of them fail the -Werror builds:

- json_pointer.hpp (#5836): size_t += int under GCC -Wsign-conversion
- binary_reader.hpp (#5839): char_int_type passed as char under GCC
  -Wconversion
- json.hpp (#5838): @throw without a description under clang
  -Wdocumentation
- string_escape.hpp (#5836): escape_into() was inserted between escape()
  and its documentation, so the @return was attached to a void function
- unit-diagnostics.cpp (#5832): clang -Wnrvo in a lambda without an
  explicit return type
- unit-to_json-rvalue.cpp (#5837): GCC -Wnoexcept (C++20) and clang
  -Wunneeded-member-function on the test type

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

include/nlohmann/detail/input/binary_reader.hpp

@@ -4272,7 +4272,7 @@ class binary_reader
     }
 
     JSON_HEDLEY_NEVER_INLINE
-    bool bson_unsupported_type_error(const std::size_t position, const char element_type) const
+    bool bson_unsupported_type_error(const std::size_t position, const char_int_type element_type) const
     {
         const std::string cr_str = hex_byte(static_cast<std::uint8_t>(element_type));
         return sax->parse_error(position, cr_str,