lispy-inline.el (lispy-describe-inline): handle large docs

* lispy-inline.el (lispy-describe-inline): Use `describe-function' if
  the window height isn't large enough to fit the inline doc whole.
  (lispy--join-pad): Rename from `lispy--pad-string', takes a list
  of strings as first argument.
  (lispy--show): Return t on success, or nil when window isn't tall
  enough to display str.
  (lispy-splice): Fix doc.