Reject integer lengths in json_document::parse

parse(ptr, len) compiled: len converted to allow_exceptions, and ptr was
read as a C string, past the end of a buffer without a terminating NUL.
Delete the overloads of parse, parse_copy, accept, and read that take an
integer other than bool where the flags are expected.

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

docs/mkdocs/docs/api/basic_json_document/accept.md

@@ -43,6 +43,11 @@ input's own copy (for inputs that are always read into a buffer) throws.
 
 Linear in the length of the input.
 
+## Notes
+
+An integer argument that is not a `#!cpp bool` where the flags are expected, such as `#!cpp accept(ptr, len)`, does not
+compile; see [`parse`](parse.md#notes).
+
 ## Examples
 
 ??? example