doc: modifies the GC section (#449)

Signed-off-by: Takeshi Yoneda <[email protected]>

doc/OVERVIEW.md

@@ -462,7 +462,10 @@ Internally, `runtime.GC` is called whenever the heap runs out (see
 [1](https://tinygo.org/lang-support/#garbage-collection),
 [2](https://github.com/tinygo-org/tinygo/blob/v0.14.1/src/runtime/gc_conservative.go#L218-L239)) in TinyGo.
 
-TinyGo allows us to disable GC, but we cannot do that since internally we need to use maps (implicitly causes allocation) for saving the Virtual Machine's state. Theoretically, we can implement our own GC algorithms tailored for proxy-wasm through `alloc(uintptr)` [interface](https://github.com/tinygo-org/tinygo/blob/v0.14.1/src/runtime/gc_none.go#L13) with `-gc=none` option. This is a future TODO.
+TinyGo allows us to disable GC, so theoretically, we can implement our own GC algorithms tailored for Proxy-Wasm through with `-gc=custom` and the build tag `-tags=custommalloc`. 
+For example, create an arena for each context and free the arena when the context is destroyed. However, it is difficult to implement that 
+since internally we need to use global maps (implicitly causes allocation outside the context scope, globally) for saving the Virtual Machine's state (e.g. context id to context implementation mapping).
+So this is not implemented yet, and that is a future TODO.
 
 ## `recover` not implemented