Added documentation for multiple entities on CRUD and Netconf services

[ygorelik]

Added documentation for multiple entities on CRUD and Netconf services for Python and C++.

Improved API for EntityCollection class

[codecov]

Codecov Report

Merging #726 into master will decrease coverage by 0.01%.
The diff coverage is 83.72%.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
- Coverage   80.46%   80.45%   -0.02%     
==========================================
  Files         170      170              
  Lines       16268    16288      +20     
  Branches     1470     1473       +3     
==========================================
+ Hits        13090    13104      +14     
- Misses       2820     2824       +4     
- Partials      358      360       +2

Impacted Files	Coverage Δ
sdk/python/core/tests/test_sanity_netconf.py	97.07% <100%> (+0.01%)	⬆️
sdk/python/core/ydk/services/netconf_service.py	72.11% <50%> (ø)	⬆️
sdk/python/core/ydk/services/crud_service.py	85% <60%> (ø)	⬆️
sdk/python/core/ydk/types/py_types.py	78% <75.67%> (-0.81%)	⬇️
sdk/python/core/tests/test_ydk_types.py	97.89% <97.14%> (+0.09%)	⬆️

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