docs(skills): document --highlights flag in firecrawl-search

[RichardBray]

Summary

• firecrawl search --help already exposes --highlights / --no-highlights (added alongside the new relevance-model search excerpts), but the firecrawl-search skill doc never documented them.
• Adds the flags to the options table and a tip explaining that highlights are on by default and backed by the server-side relevance model described in https://www.firecrawl.dev/blog/introducing-our-most-accurate-search-yet.

Test plan

• Confirmed via firecrawl search --help on CLI 1.19.27 that both flags exist and match the doc wording.
• Docs render correctly in skill viewers (markdown table + list only, no other change).

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