Enhance grok-code-fast-1 model description

Added prompt caching detail to the grok-code-fast-1 model description.

Author: mrubens

docs/providers/xai.md

@@ -39,7 +39,7 @@ xAI is the company behind Grok, a large language model known for its conversatio
 Roo Code supports the following xAI Grok models:
 
 ### Current Models
-* `grok-code-fast-1` (Default) - xAI's Grok Code Fast model with 262K context window, optimized for reasoning and coding tasks
+* `grok-code-fast-1` (Default) - xAI's Grok Code Fast model with 262K context window and prompt caching, optimized for reasoning and coding tasks
 * `grok-4` - xAI's Grok-4 model with 262K context window, image support, and prompt caching
 * `grok-3` - xAI's Grok-3 model with 128K context window and prompt caching
 * `grok-3-fast` - xAI's Grok-3 fast model with 128K context window and prompt caching