[V] Update the docstring for make-char

[Stephen J. Turnbull]

VETO

Aidan, parts of the documentation change are just plain wrong.  It is
true that only the low seven bits are used, but "octet" *is* the
correct term, as all of these standards are *on-the-wire* standards
that presume octet streams as the fundamental representation.

Also, the editorial commentary, such as references to the
unfortunate difference from MIME, doesn't belong in docstrings.
Docstrings should explain how to use the function to somebody who
already knows how XEmacs works, not try to explain how XEmacs works.