docs: Fix stale and inaccurate documentation

[vdusek]

Fixes four documentation inaccuracies surfaced by the engineering audit:

• README.md — BeautifulSoupCrawler no longer defaults to HttpxHttpClient. Updated the stated default to ImpitHttpClient, matching BasicCrawler.
• Dataset.push_data — the docstring claimed a hard 9MB per-object limit, but no local storage client enforces any size limit. Clarified that local clients impose no limit and the 9MB per-object cap applies only to remote clients such as the Apify platform.
• EnqueueLinksKwargs.strategy — the docstring described only link extraction (enqueue_links), but the field is inherited by add_requests, which filters explicitly provided requests. Reworded to cover both, and corrected the default note (same-hostname is the default for enqueue_links).
• Introduction tutorial — the "Limit your crawls" section sets max_requests_per_crawl=10 but the prose said "after the 20th request". Fixed to "10th".

Docstring and prose only, no behavior change.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (c864b96) to head (0f950af).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2081   +/-   ##
=======================================
  Coverage   93.47%   93.48%           
=======================================
  Files         181      181           
  Lines       12552    12552           
=======================================
+ Hits        11733    11734    +1     
+ Misses        819      818    -1     

Flag	Coverage Δ
unit	93.48% <100.00%> (+<0.01%)	⬆️

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