[MERGE] Small doc improvements, mostly adding See Also references

[Eric Siegerman]

>> +    are currently unknown and not ignored.
>
> [criticism that "unknown" implies "not ignored"; response
> that the nuance is subtle, and should be elaborated]

How about:
    are currently unknown -- that is, unversioned and not ignored.
or, factoring out the negation:
    are currently unknown -- that is, neither versioned nor ignored.

I'm six-of-one-half-dozen-of-the-other as to which of those is
less confusing.

  - Eric