Clarify (elisp) insert-file-contents with BEG or END not on character boundary

Clarify (elisp) insert-file-contents with BEG or END not on character boundary

* doc/lispref/files.texi (Reading from files): When the argument BEG or END to
insert-file-contents are at a byte position not at a character boundary,
clarify that raw bytes get inserted, and how to handle this awkwardness in
Lisp.  Also clarify that insert-file-contents-literally is intended to insert
raw bytes into the buffer.  Fix the outdated example that states it inserts
500 characters, when it actually inserts 500 bytes.