refactor(gles): document Buffer::map_flags (gfx-rs#8455)

Author: ErichDonGubler

wgpu-hal/src/gles/mod.rs

@@ -345,6 +345,7 @@ pub struct Buffer {
     raw: Option<glow::Buffer>,
     target: BindTarget,
     size: wgt::BufferAddress,
+    /// Flags to use within calls to [`Device::map_buffer`](crate::Device::map_buffer).
     map_flags: u32,
     data: Option<Arc<MaybeMutex<Vec<u8>>>>,
     offset_of_current_mapping: Arc<MaybeMutex<wgt::BufferAddress>>,