Add "sifted colimits" and "reflexive coequalizers"

[ykawase5048]

I have added "sifted colimits" and "reflexive coequalizers" to the list of properties.

[ScriptRaccoon]

Thanks!

I have made some small adjustments (27d2be7).

Can you please add the information if FreeAb has reflexive coequalizers?

Also, if the walking pair ${0 \rightrightarrows 1}$ has sifted colimits?

There are some other unknowns after adding the new properties, but I think these examples should be filled. They are doable.

[ykawase5048]

@ScriptRaccoon
The commit 95640cf covers FreeAb, and 752a231 covers the walking pair.
For the latter, a shorter proof may exist...

[ScriptRaccoon]

Thanks a lot!

I like your refinement of complete_characterization.

btw I fixed a small typo (which I created, not your fault)