gpu: document implementation initialization

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu/src/dawn_impl.zig

@@ -8,6 +8,8 @@ const c = @cImport({
 var procs: c.DawnProcTable = undefined;
 
 /// A Dawn implementation of the gpu.Interface, which merely directs calls to the Dawn proc table.
+///
+/// Before use, it must be `.init()`ialized in order to set the global proc table.
 pub const Interface = gpu.Interface(struct {
     pub fn init() @This() {
         procs = c.machDawnGetProcTable();

gpu/src/interface.zig

@@ -2,6 +2,8 @@ const gpu = @import("main.zig");
 
 /// The gpu.Interface implementation that is used by the entire program. Only one may exist, since
 /// it is resolved fully at comptime with no vtable indirection, etc.
+///
+/// Depending on the implementation, it may need to be `.init()`ialized before use.
 pub const Impl = blk: {
     if (@import("builtin").is_test) {
         break :blk StubInterface;