doc: Clarify description of `FT_Stream_IoFunc`.
The existing documentation comments on `FT_Stream_IoFunc` hinted at the dual
seek/read behavior required for custom stream implementations to work, but
it didn't explicitly explain it. Without looking at the internals of
FreeType, it was easy for someone to assume their implementation should
handle both seek and read operations all the time. If this is done, you get
a variety of errors, but mostly just `Unknown_File_Format` (error code
0x02).
1 file changed
