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/string_escape.hpp

@@ -17,18 +17,7 @@ NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
-/*!
- * @brief string escaping as described in RFC 6901 (Sect. 4)
- * @param[in] s string to escape
- * @return    escaped string
- *
- * Note the order of escaping "~" to "~0" and "/" to "~1" is important.
- *
- * The string is rebuilt in a single pass, appending whole runs between the
- * characters that need escaping, while repeated replace() calls would move
- * the tail of the string once per escaped character. @ref escape_into appends
- * the escaped string to an existing one.
- */
+/// append @a s to @a out, escaped like @ref escape does
 template<typename StringType>
 inline void escape_into(StringType& out, const StringType& s)
 {
@@ -47,6 +36,18 @@ inline void escape_into(StringType& out, const StringType& s)
     out.append(s.data() + run, size - run);
 }
 
+/*!
+ * @brief string escaping as described in RFC 6901 (Sect. 4)
+ * @param[in] s string to escape
+ * @return    escaped string
+ *
+ * Note the order of escaping "~" to "~0" and "/" to "~1" is important.
+ *
+ * The string is rebuilt in a single pass, appending whole runs between the
+ * characters that need escaping, while repeated replace() calls would move
+ * the tail of the string once per escaped character. @ref escape_into appends
+ * the escaped string to an existing one.
+ */
 template<typename StringType>
 inline StringType escape(const StringType& s)
 {