docs: clarify LSP tool enablement

[dannyward630]

Summary

• clarify that OPENCODE_EXPERIMENTAL_LSP_TOOL exposes the experimental lsp tool
• point readers to the lsp config option for enabling and configuring LSP servers

Closes #30954

Verification

• bun run build from packages/web
• full pre-push bun turbo typecheck

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The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found

PR #32876: docs: clarify LSP default enablement
#32876

This PR appears to be closely related to the current PR #32885. Both are documentation PRs addressing LSP enablement clarification with very similar titles. They likely cover overlapping or adjacent documentation updates about LSP configuration and enablement.

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