mirage-crypto-rng-unix: more documentation

rng/unix/mirage_crypto_rng_unix.mli

@@ -12,13 +12,24 @@ val initialize : ?g:'a -> 'a Mirage_crypto_rng.generator -> unit
 (** [getrandom size] returns a buffer of [size] filled with random bytes. *)
 val getrandom : int -> string
 
+(** A generator that opens /dev/urandom and reads from that file descriptor
+    data whenever random data is needed. The file descriptor is closed in
+    [at_exit]. *)
 module Urandom : Mirage_crypto_rng.Generator
 
+(** A generator using [getrandom(3)] on Linux, [getentropy(3)] on BSD and macOS,
+    and [BCryptGenRandom()] on Windows. *)
 module Getentropy : Mirage_crypto_rng.Generator
 
+(** [use_default ()] initializes the RNG [Mirage_crypto_rng.default_generator]
+    with [Urandom] or resorts to [Getentropy] if the urandom failed to open the
+    /dev/urandom device. *)
 val use_default : unit -> unit
-(** [use_default ()] initializes with [Urandom] or resorts to [Getentropy] otherwise. *)
 
+(** [use_dev_random ()] initializes the RNG [Mirage_crypto_rng.default_generator]
+    with the [Urandom] generator. *)
 val use_dev_urandom : unit -> unit
 
+(** [use_getentropy ()] initializes the RNG [Mirage_crypto_rng.default_generator]
+    with the [Getentropy] generator. *)
 val use_getentropy : unit -> unit