[API-899] Update SqlService API documentation

[mdumandag]

• Removed the warning about the BETA status
• Similar to Java client, simplified the API doc as most of the
  things we mention here is already mentioned in the actual
  SQL documentation
• Add a warning about enabling Jet and adding hazelcast-sql
  to the member's classpath
• And other minor updates.

[codecov-commenter]

Codecov Report

Merging #479 (e3d92c1) into master (e9dfb0a) will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   95.16%   94.81%   -0.36%     
==========================================
  Files         345      346       +1     
  Lines       17711    17730      +19     
==========================================
- Hits        16855    16810      -45     
- Misses        856      920      +64     

Impacted Files	Coverage Δ
hazelcast/sql.py	97.30% <ø> (ø)
...otocol/codec/client_authentication_custom_codec.py	50.00% <0.00%> (-23.81%)	⬇️
hazelcast/reactor.py	80.09% <0.00%> (-9.23%)	⬇️
hazelcast/listener.py	91.82% <0.00%> (-2.52%)	⬇️
hazelcast/statistics.py	88.62% <0.00%> (-2.40%)	⬇️
hazelcast/protocol/client_message.py	93.52% <0.00%> (-1.77%)	⬇️
hazelcast/connection.py	91.71% <0.00%> (-1.59%)	⬇️
hazelcast/invocation.py	93.86% <0.00%> (-1.54%)	⬇️
hazelcast/six.py	66.95% <0.00%> (-0.22%)	⬇️
...ast/protocol/codec/map_remove_interceptor_codec.py	100.00% <0.00%> (ø)
... and 2 more

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