Update Resolvers description in Client reference docs

[JanBednarik]

What do these changes do?

From the docs it seems like asynchronous Resolver is used by default when aiodns is installed. It's not true since: 9fbb7d7 This change should make it clear.

Are there changes in behavior for the user?

No.

Related issue number

No.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #3642 into master will decrease coverage by 3.49%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #3642     +/-   ##
=========================================
- Coverage    97.9%   94.41%   -3.5%     
=========================================
  Files          43       43             
  Lines        8556     8555      -1     
  Branches     1376     1375      -1     
=========================================
- Hits         8377     8077    -300     
- Misses         74      359    +285     
- Partials      105      119     +14

Impacted Files	Coverage Δ
aiohttp/worker.py	6.55% <0%> (-90.99%)	⬇️
aiohttp/resolver.py	46.55% <0%> (-53.45%)	⬇️
aiohttp/web_fileresponse.py	66.66% <0%> (-29.89%)	⬇️
aiohttp/tcp_helpers.py	63.33% <0%> (-26.67%)	⬇️
aiohttp/typedefs.py	91.3% <0%> (-8.7%)	⬇️
aiohttp/pytest_plugin.py	89.79% <0%> (-7.49%)	⬇️
aiohttp/locks.py	92.85% <0%> (-7.15%)	⬇️
aiohttp/web_runner.py	91.7% <0%> (-6.22%)	⬇️
aiohttp/__init__.py	94.44% <0%> (-5.56%)	⬇️
aiohttp/http_parser.py	93.11% <0%> (-4.82%)	⬇️
... and 11 more

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[asvetlov]

Thanks