Don't over-truncate erc-timestamp-format-left

Don't over-truncate erc-timestamp-format-left

* lisp/erc/erc-stamp.el (erc-timestamp-format-left): Fix typo in doc
string and mention that changing the value mid-session requires
cycling the minor mode.
(erc-echo-timestamp-format): Add Custom :tag for choices.
(erc-stamp--date-format-end): Revise doc string.
(erc-stamp--format-date-stamp): Fix bug involving erroneous truncation
parameter for `substring' when `erc-timestamp-format-left' doesn't end
in a newline.  Thanks to Emanuel Berg for catching this.
(erc-stamp-prepend-date-stamps-p) Revise doc string.
(erc-insert-timestamp-left-and-right): Add comment regarding
compatibility concession.  (Bug#60936)