render/gles2: reword wlr_gles2_texture comments
These comments were a bit misleading:
- "GL_TEXTURE_2D == mutable": not really, imported non-external-only DMA-BUFs would also use this target, but are not mutable.
- "Only affects target == GL_TEXTURE_2D": same here.
- "If imported from a wlr_buffer": not really, would be NULL if imported from a shm wlr_buffer.
Adjust these comments to better reflect reality and adjust the check in gles2_texture_update_from_buffer().