gst-libs: gl: Fix documentation typo and clarify gl_memory_texsubimage

Followup for gstreamer-rs!581 (merged)

gst_gl_memory_read_pixels reads pixels from GLMemory into the pointer, effectively writing to it. This is opposite from gst_gl_memory_texsubimage which reads texture data from read_pointer into GLMemory.

Both cases are clarified by changing read_pointer to write_pointer, and explaining what gst_gl_memory_texsubimage does in addition to referring back to gst_gl_memory_read_pixels.