Add an error_handler parameter for UTF-8 to the binary readers and writers (#5746)

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

docs/mkdocs/docs/features/binary_formats/cbor.md

@@ -192,12 +192,17 @@ The library maps CBOR types to JSON value types as follows:
 !!! warning "Ill-formed UTF-8 in text strings"
 
     [RFC 8949, Section 3.1](https://www.rfc-editor.org/rfc/rfc8949.html#section-3.1) requires CBOR text strings (major
-    type 3) to be valid UTF-8, but leaves it up to the decoder whether to enforce this. This library does not:
-    `from_cbor()` accepts a text string (object keys included) whose bytes are not valid UTF-8 and hands them back
-    unchanged. However, [`dump()`](../../api/basic_json/dump.md) still requires valid UTF-8 and throws
-    [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) for such a value, unless an error handler is
-    passed that replaces or ignores the ill-formed bytes. By default, `to_cbor()` writes such a value back unchanged; if
-    [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) is enabled, it throws the same exception
+    type 3) to be valid UTF-8, but leaves it up to the decoder whether to enforce this, so checking is opt-in: with the
+    [`error_handler`](../../api/basic_json/from_cbor.md) parameter left at `keep` (the default), `from_cbor()` accepts a
+    text string (object keys included) whose bytes are not valid UTF-8 and hands them back unchanged. Passing
+    `error_handler_t::strict` makes `from_cbor()` check and throw
+    [`parse_error.113`](../../home/exceptions.md#jsonexceptionparse_error113) for ill-formed UTF-8, and
+    `replace`/`ignore` sanitize the string instead of keeping it. However, [`dump()`](../../api/basic_json/dump.md)
+    still requires valid UTF-8 and throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) for a
+    value read with the default `keep` handler, unless an error handler is passed that replaces or ignores the
+    ill-formed bytes. `to_cbor()`'s own [`error_handler`](../../api/basic_json/to_cbor.md) parameter defaults to `keep`,
+    so such a value is written back unchanged; with `strict` (the default if
+    [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) is enabled), it throws the same exception
     instead. Byte strings (major type 2) are unaffected, since they are not required to hold text.
 
 !!! warning "Tagged items"