Improve format-spec documentation (bug#41571)
* doc/lispref/text.texi (Interpolated Strings): Move from here... * doc/lispref/strings.texi (Custom Format Strings): ...to here, renaming the node and clarifying the documentation. (Formatting Strings): End node with sentence referring to the next one. * lisp/format-spec.el (format-spec): Clarify docstring.