Add documentation for methods of coping with narrow lines

[alerque]

Originally posted by @jodros in #2001 (comment)

As has come up in the above issue and many other times, it is not always apparent what methods are available to adjust settings to cope with narrow text width. The typesetter defaults are much more appropriate to comfortably sized book line widths and absolutely do not gracefully cope with narrow widths out of the box. There is no one right solution, but there are a lot of knobs to fiddle with to find a nice compromise. The docs should probably have a section devoted to this specifically. We mention it in a few places already, but the possible adjustments are mentioned helter-skelter wherever they appear in other contexts.

[Omikhleia]

See https://github.com/sile-typesetter/sile/wiki/FAQ#some-lines-are-overflowing-my-page-margin

Maybe some of it can be re-used.