Improve documentation of registers

Improve documentation of registers

* doc/lispref/text.texi (Registers): Document buffers in
registers.  Mention "frameset" as another name for "frame
configuration".
* doc/emacs/regs.texi (Registers, Configuration Registers)
(File and Buffer Registers): Clarify and improve wording.  Add
cross-references and indexing.
(Configuration Registers): Rename the section to a more accurate
name.  (Bug#64354)

* lisp/register.el (jump-to-register, point-to-register)
(register-alist, frame-configuration-to-register): Doc fixes.
(Bug#64353)