Document that random_get can block.

docs/WASI-api.md

@@ -954,6 +954,9 @@ Inputs:
 
 Write high-quality random data into a buffer.
 
+This function blocks when the implementation is unable to immediately
+provide sufficient high-quality random data.
+
 This function may execute slowly, so when large mounts of random
 data are required, it's advisable to use this function to seed a
 pseudo-random number generator, rather than to provide the