Improve byLine doc

Move byLine examples from ByLine to byLine.
Fix LinesReader typo.
Explain that lines will not persist when using a mutable type for Char.
Document parameters.
Make byLine return an opaque type

File.ByLine is still visible for backwards compatibility, but no longer
documented.
s/forward range/input range/ plus minor fixes
Merge pull request #1408 from ntrel/byLine-doc

Improve byLine doc