We can have the previous behaviour by just passing the docstring to
`info -placement menu`.

Also, moved the "Markup strings" section to faces.asciidoc, since it wasn't
anything to do with expansions, and updated various hyperlinks to point at the
new location.

Automatic reparsing of %sh{...}, while convenient in many cases,
can be surprising as well, and can lead to security problems:

'echo %sh{ printf "foo\necho bar" }' runs 'echo foo', then 'echo bar'.
we make this danger explicit, and we fix the 'nop %sh{...}' pattern.

To reparse %sh{...} strings, they can be passed to evaluate-commands,
which has been fixed to work in every cases where %sh{...} reparsing
was used..

Fixes #1891