clarify doc for stan() on whether Stan or R does random initialization

[bob-carpenter]

An issue came up from Ryan Brackney on stan-users about which seed controls the initialization if init="random" is specified in a call to stan().

Also, the "One of" to start the comment on init is confusing because it can actually be a positive double value other than 0, as you indicate later in the doc with init_r.

• clarify whether R/Stan does random init
• reword init doc to make it clearer that you can use a positive double value

[maverickg]

there is a commit f00309b

please see if it's clearer or please revise it.

you cannot use double for init.

[jgabry]

closing outdated issue