add docs for selected_call (#2028)

<!-- ELLIPSIS_HIDDEN -->



> [!IMPORTANT]
> Add documentation for `selected_call` property in `FunctionLog` class
in `collector.mdx`.
> 
>   - **Documentation**:
> - Adds `selected_call` property to `FunctionLog` class in
`collector.mdx`.
> - Describes `selected_call` as the call used by BAML for parsing the
response among multiple retries or fallbacks.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for ef4cbaa. You can
[customize](https://app.ellipsis.dev/BoundaryML/settings/summaries) this
summary. It will automatically update as commits are pushed.</sup>

<!-- ELLIPSIS_HIDDEN -->

Author: Ben-Epstein