[doc] Type1 GX (sfnt-wrapped Type1) is not supported.

* docs/formats.txt: Clarify the reference of Type1 GX,
and state that this format is not supported.  This
is because the content `TYP1' table is not exactly same
with the Type1 font format specification; no eexec
encryption is used.  For detail and concrete examples,
see the analysis on:

#1231