doc: Document `co_serializable()`.

doc/examples/test_serialization.cpp

@@ -75,6 +75,11 @@ auto APU::leaf() -> void {
 }
 
 auto main() -> int {
+  if(!co_serializable()) {
+    printf("This implementation does not support serialization\n");
+    return 1;
+  }
+
   Memory::buffer = (uint8_t*)mmap(
     (void*)0x10'0000'0000, 2 * 65536,
     PROT_READ | PROT_WRITE | PROT_EXEC,

doc/usage.md

@@ -126,6 +126,19 @@ Passing handle of active cothread to this function is not allowed.
 
 Passing handle of primary cothread is not allowed.
 
+## co_serializable
+
+```c
+int co_serializable(void);
+```
+
+Returns non-zero if the implementation keeps the entire coroutine state in the
+buffer passed to `co_derive()`. That is, if `co_serializable()` returns
+non-zero, and if your cothread does not modify the heap or any process-wide
+state, then you can "snapshot" the cothread's state by taking a copy of the
+buffer originally passed to `co_derive()`, and "restore" a previous state
+by copying the snapshot back into the buffer it came from.
+
 ## co_switch
 ```c
 void co_switch(cothread_t cothread);