Document IOContext keys & values

[timholy]

As far as I can tell, IOContext is documented in these places:

• http://docs.julialang.org/en/latest/stdlib/io-network/?highlight=iocontext
• http://docs.julialang.org/en/latest/manual/networking-and-streams/?highlight=iocontext#io-output-contextual-properties

This doesn't seem like enough information to guide folks in how to use it; I just grep base/ every time I need to remind myself, but we probably don't want to encourage that. At a bare minimum, it seems that :limit, :limit_output, :compact, and :multiline should be clearly described.

[tkelman]

x-ref #17545 which is adding a bit but not quite enough for this to be complete IMO

[JeffBezanson]

Dup of #16763.