Rework infinitesimals docstring.
Fix dsolve reference under Hint Functions.
Fix logcombine reference.
Rework awkward language under constantsimp.
Fix references to several ode functions.
Shorten function references under "See also" headers.
Rewrap with tw=78:et.
Revert "Shorten function references under "See also" headers."

This reverts commit 203798670ce80881555eee5be9ce625454cc8f60.
Fix reference to separatevars.
Rewrap lines tw=78.
Rephrase "Hint Functions" and "User Functions" intros.
Merge upstream commits.
Merge pull request #2329 from alhirzel/ode-py-docs-take-two

More fixes for ode.py documentation