PROTON-2402: Clarify proton docs for on_sendable, prefetch for Messag…

[NS-SlaFleur]

…ingHandler, send for Sender and thread-safety for Container.

https://issues.apache.org/jira/browse/PROTON-2402

[codecov-commenter]

Codecov Report

Merging #324 (5c834cf) into main (a3d1ef4) will increase coverage by 19.68%.
The diff coverage is n/a.

❗ Current head 5c834cf differs from pull request most recent head 2a10d7a. Consider uploading reports for the commit 2a10d7a to get more accurate results

@@             Coverage Diff             @@
##             main     #324       +/-   ##
===========================================
+ Coverage   68.66%   88.34%   +19.68%     
===========================================
  Files         351       47      -304     
  Lines       70321     2394    -67927     
===========================================
- Hits        48285     2115    -46170     
+ Misses      22036      279    -21757     

Impacted Files	Coverage Δ
cpp/include/proton/source.hpp
cpp/examples/message_properties.cpp
build/c/src/core/util.c
cpp/src/error.cpp
cpp/include/proton/internal/object.hpp
python/proton/_selectable.py
cpp/src/map.cpp
cpp/include/proton/binary.hpp
c/tests/connection_driver_test.cpp
c/src/handlers/handshaker.c
... and 291 more

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[NS-SlaFleur]

@NS-MNellen will take over this PR if there are any questions.

[astitcher]

@ssorj Could you look at these doc changes and see if you think this is an improvement.