SI-10225 Either docs have more words and code
SI-10225 Either docs have more words and code

Add more words to explain the behavior of `Either`
in for comprehensions.

Simplify pattern matches where biased.

Change type parameters to the `[A1, B1]` convention.

scala/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,martijnhoekstra/scala,martijnhoekstra/scala,martijnhoekstra/scala,lrytz/scala,lrytz/scala,scala/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,lrytz/scala,scala/scala,lrytz/scala,scala/scala,lrytz/scala