Added information about nature of block keys (#1892)

Summary:
**Summary**

Added a note letting users know the block keys are strictly alphanumeric and using the `generateRandomKey` function to generate block keys is a good approach.

Any changes needed can be mentioned and will be implemented in a jiffy!

Issue in focus: #1843
Pull Request resolved: #1892

Differential Revision: D10303263

fbshipit-source-id: 717baed9d04b91acb4e73b936f3e367b4fb11f04

Author: Prateek Nayak

docs/APIReference-ContentBlock.md

@@ -161,7 +161,7 @@ supplied text.
 ```
 getKey(): string
 ```
-Returns the string key for this `ContentBlock`.
+Returns the string key for this `ContentBlock`. Block keys are alphanumeric string. It is recommended to use `generateRandomKey` to generate block keys.
 
 ### getType()