Simplify erc-tab interface

Simplify erc-tab interface

* etc/ERC-NEWS: Improve section mentioning `erc-tab'.
* lisp/erc/erc-button.el (erc-button-alist): Restore a commented out
version of the "<URL ...>" entry, which was dropped in 5adda2f4683
"Revise FORM-as-function interface in erc-button-alist" for being
redundant.  This may help unlikely objectors better adapt to the
churn.
(erc-button-next, erc-button-previous): Don't bother accommodating raw
prefix arguments; expect numeric arguments only.
* lisp/erc/erc.el (erc-tab): Change interactive spec to lowercase
"p".  (Bug#60933)