[FEATURE]: Document pruning functionality

[Sewer56]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

There is currently no documentation for the pruning functionality available in the documentation.

Might be worth noting and explaining:

• What pruning is, and in what contexts it is used within this project
  • i.e. Removing tool call results to free up context
• How to use pruning (e.g., configuration, commands, options)
  • Can only use OPENCODE_DISABLE_PRUNE env var right now.
• Any important caveats or side-effects
  • e.g. Invalidates token cache

And why the used context keeps randomly flickering between 2 values (it's pruning).