Add a description of the expectation of request_callback timing (#422)

* Add a description of the expectation of request_callback timing

Without making a requirement, indicate the intent of the timing.

* Add an apostrophe

* Add host can starve feedback from alex

* more review feedback

* notjusthosts

include/clap/host.h

@@ -35,6 +35,12 @@ typedef struct clap_host {
    void(CLAP_ABI *request_process)(const struct clap_host *host);
 
    // Request the host to schedule a call to plugin->on_main_thread(plugin) on the main thread.
+   // This callback should be called as soon as practicable, usually in the host application's next
+   // available main thread time slice. Typically callbacks occur withink 33ms / 30hz.
+   // Despite this guidance, plugins should not make assumptions about the exactness of timing for
+   // a main thread callback, but hosts should endeavour to be prompt. For example, in high load situations
+   // the environment may starve the gui/main thread in favor of audio processing, leading to substantially
+   // longer latencies for the callback than the indicative times given here.
    // [thread-safe]
    void(CLAP_ABI *request_callback)(const struct clap_host *host);
 } clap_host_t;