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>

tests/src/unit-to_json-rvalue.cpp

@@ -39,20 +39,15 @@ struct tracked
     std::string payload;
     bool moved_from = false;
 
-    explicit tracked(std::string p) : payload(std::move(p)) {}
+    explicit tracked(std::string p) noexcept : payload(std::move(p)) {}
     tracked(const tracked&) = default;
     tracked& operator=(const tracked&) = default;
     tracked(tracked&& other) noexcept : payload(std::move(other.payload)), moved_from(other.moved_from)
     {
         other.moved_from = true;
     }
-    tracked& operator=(tracked&& other) noexcept
-    {
-        payload = std::move(other.payload);
-        moved_from = other.moved_from;
-        other.moved_from = true;
-        return *this;
-    }
+    // not needed by the tests (and an unused definition fails -Wunneeded-member-function)
+    tracked& operator=(tracked&&) = delete;
     ~tracked() = default;
 };