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/json_pointer.hpp

@@ -1077,7 +1077,7 @@ class json_pointer
         std::size_t token_count = 0;
         for (std::size_t i = 0; i < size; ++i)
         {
-            token_count += (reference_string[i] == '/') ? 1 : 0;
+            token_count += static_cast<std::size_t>(reference_string[i] == '/');
         }
         result.reserve(token_count);