Reorganized API documentation.

Author: kernelmethod

docs/src/api.md

@@ -1,12 +1,32 @@
 # API
 
-```@index
-```
-
 ```@meta
 CurrentModule = ChaChaCiphers
 ```
 
+## Keystream types
+
+```@autodocs
+Modules = [ChaChaCiphers]
+Filter = t -> (t isa DataType && t <: ChaChaCiphers.AbstractChaChaStream) ||
+  t in (ChaCha12Stream, ChaCha20Stream, CUDAChaCha12Stream, CUDAChaCha20Stream)
+```
+
+## Keystream helpers
+
+```@autodocs
+Modules = [ChaChaCiphers]
+Filter = t -> t in (getstate, ChaChaCiphers.ChaChaState)
+```
+
+## Encryption and decryption
+
 ```@autodocs
 Modules = [ChaChaCiphers]
+Filter = t -> t in (encrypt!, decrypt!)
+```
+
+## Index
+
+```@index
 ```