callback-docs-fix

[lavinigam-gcp]

callback-docs-fix

Fixes:

• In the intro to callbacks, added more description for agent callbacks to make it more clear - as pointed out in: Clarification on Callbacks #156
• The before and after agent code block is modified to showcase proper functioning with states
• The flow diagram is fixed. Pointed out in this: bug: callback flow diagram shows incorrect callback functions for tools #111