clarify empty array expression

[avehtari]

Fixes #621

The restriction on empty array expression is mentioned later, but it is easy to miss, so clarified it now also earlier with link to the later section. Also added the suggested way to create an empty array.

[bob-carpenter]

I suggested a more extensive subsection expanding on what was submitted.

[bob-carpenter]

The real and int should be escaped as real and int. (back ticks)

[bob-carpenter]

I think it'd be worth pulling out a subsection and highlighting the empty array can be any type, not just a scalar.

Empty arrays {-}

The empty array expression ({ }) is not allowed. See more about restrictions on array expressions in subsection Restrictions on values.

If needed, it is also possible to create an empty array with the expression

rep_array(e, 0)

where the first expression e determines the type of the array. For example, rep_array(0.0, 0) returns an empty real array of type real[], whereas rep_array({123}, 0) returns an empty two dimensional integer array of type int[ , ]. Only the type of the first argument is used, so the integer arrays {123} and {0} produce equivalent values.

[avehtari]

Github shows the markdown in the comment so, that I can't see how many # you used for the subsection "Empty arrays". Also I'm not certain where you would like to have that subsection.

[avehtari]

Added also instruction how to create empty vectors and matrices

[bob-carpenter]

Sorry about that---you can look at the source with the "edit" button. Three hashes.

The only reason I pulled it out into a section is to make it more noticeable and better indexed by Google (though I'm not sure the indexing is helped much any more by level 3 headers---it used to be).

[bob-carpenter]

Looks good. Thanks.