Change the default sampler to ParentOrElse(AlwaysOn)

[MitchellDumovic]

Discussed at the Spec SIG this morning 7/28. Resolves #728

[dyladan]

Because it is the default, you may want to move it to the top of the list. I would also consider adding a new heading "Default Sampler" that specifically calls out that this is the default sampler so it isn't hidden in a bullet list.

[MitchellDumovic]

@dyladan I pushed some changes that hopefully address the problem here. I have a slight preference for not leading with ParentOrElse as it is a more complex sampler type and the default ParentOrElse(AlwaysOn) references both ParentOrElse and AlwaysOn.

It felt more right to me to include the default header right next to the documentation of the built-in sampler types rather than adding a new higher level header, but again this is a small preference so happy to split out a separate header as well if you think that is more clear and makes the default more obvious.

[dyladan]

Suggested change

	The default sampler if one is not specified is `ParentOrElse(AlwaysOn)`.
	The default sampler if is `ParentOrElse(AlwaysOn)`.

[Oberon00]

I'm OK with this change, but IMHO it was merged far too fast. #745 suggests to wait two business days, but even without that, I think merging a non-trivial change on the same day it was proposed is unfair.

[yurishkuro]

@Oberon00 my bad. Is there some bot we can install that would block merge for a predefined "public comment" period? Similar to how WIP bot blocks the merge.