[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:

https://gitlab.freedesktop.org/freetype/freetype/-/issues/1231

docs/formats.txt

@@ -77,8 +77,8 @@ which isn't supported yet please send a mail too.
   ---     BDF     ---     ---        bdf      5005.BDF_Spec.pdf, X11
 
 
-  SFNT    PS      TYPE_1  ---        type1    Type 1 GX Font Format
-                                              (for the Mac) [3]
+  SFNT    PS      TYPE_1  ---        type1    Type 1 GX Font Format [7]
+                                              (for the Mac; not supported)
   SFNT    PS      TYPE_1  CID        cid      5180.sfnt.pdf (for the Mac) [3]
   SFNT    PS      CFF     ---        cff      OT spec, 5176.CFF.pdf
                                               (`OTTO' format)
@@ -198,6 +198,12 @@ which isn't supported yet please send a mail too.
 [6] Supported  font  formats  are   TrueType  and  OpenType  fonts  as
     defined in the OpenType specification 1.6 and newer.
 
+[7] `The Type 1 GX Font Format' (dated 1995-09-27)  was distributed in
+    Apple Developer CD-ROM in those days.  The content of `TYP1' table 
+    is a PostScript Type 1 font without the eexec encryption.  Current
+    versions of FreeType don't not support this format,  but FontForge
+    can load it.
+
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