Fix documentation of offsets in text glyph serialization format (#5606) Offsets are serialized when only one of `x` and `y` is non-zero too: https://github.com/harfbuzz/harfbuzz/blob/7c2105a52e6b50e448064e82c760dad0b5f4bdb9/src/hb-buffer-serialize.cc#L302-L303