Follow each binary format's UTF-8 rule: strict writers (CBOR/UBJSON/BJData/BSON), lenient readers (#5741)

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

docs/mkdocs/docs/home/exceptions.md

@@ -340,8 +340,9 @@ An unexpected byte was read in a [binary format](../features/binary_formats/inde
 ### json.exception.parse_error.113
 
 A string could not be read from a [binary format](../features/binary_formats/index.md): either a value that is not a
-string was read where one was required (for instance as a map key), the string's length specification is invalid, or
-the string's bytes are not valid UTF-8.
+string was read where one was required (for instance as a map key), or the string's length specification is invalid.
+The bytes of a string itself are not checked for valid UTF-8 on read; see the ill-formed UTF-8 notes on the
+individual [binary format](../features/binary_formats/index.md) pages for how such a string is handled afterward.
 
 CBOR and MessagePack allow map keys of any type, but JSON object keys are always strings. Maps with keys of any other
 type (for instance integers or `null`) are therefore not supported; see the notes on
@@ -364,9 +365,6 @@ type (for instance integers or `null`) are therefore not supported; see the note
     ```
     [json.exception.parse_error.113] parse error at byte 3: syntax error while parsing BJData string: string length must not be negative
     ```
-    ```
-    [json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: invalid string: ill-formed UTF-8 byte
-    ```
 
 ### json.exception.parse_error.114
 
@@ -749,6 +747,11 @@ The [`unflatten()`](../api/basic_json/unflatten.md) function only works for an o
 
 The [`dump()`](../api/basic_json/dump.md) function only works with UTF-8 encoded strings; that is, if you assign a `std::string` to a JSON value, make sure it is UTF-8 encoded. See the FAQ entry on [serializing untrusted or invalid UTF-8](faq.md#serializing-untrusted-or-invalid-utf-8) for background and the recommended fix.
 
+If [`JSON_STRICT_BINARY_UTF8`](../api/macros/json_strict_binary_utf8.md) is enabled, the binary writers
+[`to_cbor()`](../api/basic_json/to_cbor.md), [`to_ubjson()`](../api/basic_json/to_ubjson.md),
+[`to_bjdata()`](../api/basic_json/to_bjdata.md), and [`to_bson()`](../api/basic_json/to_bson.md) throw this exception
+for a string value or object key that is not valid UTF-8 as well.
+
 !!! failure "Example message"
 
     Calling `dump()` on a JSON value containing an ISO 8859-1 encoded string: