further cleanup to change "sampling statement" to "distribution statement" throughout the docs.

[mitzimorris]

There are a few places in the quarto markdown where "sampling statement" is broken across lines and therefore remains, e.g.,
src/stan-users-guide/user-functions.qmd lines 255-256.

There are also ~75 LaTeX index entries of the form \index{{\tt \bfseries <dist> }!sampling statement|hyperpage} across the functions-reference distribution files.