Update recommended usage in documentation.

Author: kernelmethod

docs/src/index.md

@@ -28,32 +28,18 @@ DocTestSetup = quote
 end
 ```
 
-To start generating random numbers with ChaChaCiphers, create a new keystream
-with a function like [`ChaCha20Stream`](@ref) or [`ChaCha12Stream`](@ref):
+To start generating random numbers, create a new keystream by constructing a
+[`ChaChaStream`](@ref) instance:
 
 ```jldoctest
 julia> using ChaChaCiphers
 
-julia> rng = ChaCha20Stream();
+julia> rng = ChaChaStream();
 ```
 
 This will generate a keystream using a key sampled from the operating system's
-random stream. Alternatively,  you can explicitly specify a `key` and `nonce` as
-follows:
-
-```jldoctest
-julia> key = UInt32.([
-          0xe2e39848, 0x70bb974d, 0x845f88b4, 0xb30725e4,
-          0x15c309dc, 0x72d545bb, 0x466e99e3, 0x6a759f91
-       ]);
-
-julia> nonce = UInt64(0);
-
-julia> rng = ChaCha20Stream(key, nonce);
-```
-
-After generating a keystream, you can supply it as the `rng` parameter to
-`Random` functions like `rand` and `randn`:
+random stream. You can then supply `rng` as a parameter to `Random` functions,
+like `rand` and `randn`:
 
 ```jldoctest
 julia> using Random
@@ -70,6 +56,23 @@ julia> randn(rng, Float32, 3)
 
 Review the API documentation for more details.
 
+## Advanced usage
+
+In general, we recommend instantiating `ChaChaStream` by simply calling
+`ChaChaStream()`. However, in some cases you may wish to explicitly supply a key
+to construct your stream, which you can do as follows:
+
+```jldoctest
+julia> key = UInt32.([
+          0xe2e39848, 0x70bb974d, 0x845f88b4, 0xb30725e4,
+          0x15c309dc, 0x72d545bb, 0x466e99e3, 0x6a759f91
+       ]);
+
+julia> rng = ChaCha20Stream(key);
+```
+
+In this mode, the key will act as a seed for the random number generator.
+
 ## About ChaCha
 
 ChaCha was first introduced as a variant of the Salsa20 stream cipher by Daniel