Improve API/lifecycle docs

[christophfroehlich]

The doxygen docstring of the interfaces classes was broken (see here).

• I fixed the syntax
• Removed wrong (never implemented) behavior description
• Added a lifecycle section in the sphinx docs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.54%. Comparing base (3839200) to head (cf730d0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2081      +/-   ##
==========================================
+ Coverage   89.49%   89.54%   +0.05%     
==========================================
  Files         139      139              
  Lines       15126    15124       -2     
  Branches     1283     1283              
==========================================
+ Hits        13537    13543       +6     
+ Misses       1100     1095       -5     
+ Partials      489      486       -3     

Flag	Coverage Δ
unittests	89.54% <ø> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../include/hardware_interface/actuator_interface.hpp	95.71% <ø> (ø)
...ce/include/hardware_interface/sensor_interface.hpp	96.20% <ø> (+4.95%)	⬆️
...ce/include/hardware_interface/system_interface.hpp	86.87% <ø> (ø)

... and 1 file with indirect coverage changes

[saikishor]

LGTM

[saikishor]

Thanks for the changes