Merge branch 'u/mantepse/lazy_taylor_series' of git://trac.sagemath.o…

…rg/sage into t/32345/special_functions_for_lazy_series

* 'u/mantepse/lazy_taylor_series' of git://trac.sagemath.org/sage:
  fix and document series reversion
  only get coefficient if necessary
  Refactoring we discussed at the meeting today.
  Revert "Merge branch 'u/gh-tejasvicsr1/special_functions_for_lazy_series' of git://trac.sagemath.org/sage into t/32324/lazy_taylor_series"
  Adding documentation to the user-facing class level documentation.
  Small reworkings and fixes. Adding another composition test.
  fix invert
  catch more bad input in element_constructor
  take care of negative degree of f in __call__