update documentation for callbacks

[tchaton]

What does this PR do?

In callbacks.rst, we were listing ModelHooks functions and not Callback class hooks.

This PR rectify the documentation.

Fixes # (issue)

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[codecov]

Codecov Report

Merging #4253 into master will increase coverage by 3%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #4253    +/-   ##
=======================================
+ Coverage      90%     93%    +3%     
=======================================
  Files         103     103            
  Lines        7842    7842            
=======================================
+ Hits         7053    7275   +222     
+ Misses        789     567   -222