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/read.md

@@ -49,6 +49,9 @@ whether or not the new parse succeeds; take fresh views from [`root()`](root.md)
 `input` is borrowed or owned by the same rules as [`parse()`](parse.md#notes); a document can borrow on one call and
 own on the next, since ownership is decided freshly each time.
 
+An integer argument that is not a `#!cpp bool` where the flags are expected, such as `#!cpp read(ptr, len)`, does not
+compile; see [`parse`](parse.md#notes).
+
 ## Examples
 
 ??? example