bug#62030: 29.0.60; Allow users to customize eldoc buffer separator

[Yuan Fu]

Eldoc can combine documentation from multiple sources and display them
together, however, if I’m not mistaken, it separates them with newline. It 
would be nice
if users can customize the separator to be some other things, like a
horizontal bar, eg,

(setq eldoc-source-separator
      (propertize "----------"
                  'display '(space :width text)
                  'face '(:stike-through t)))

Maybe even allow it to be a function so we can draw an appropriate
number of "-" in terminals.

Prompted by this report: https://github.com/casouri/eldoc-box/issues/61

I can do it, if everyone agrees that this is a good idea, and can come
up with a good name for the variable ;-)