Tweak syntax of extlinks to also allow ``%s`` in the link caption part.
Like for the base URL ``%s`` will be substituted with the content of the
role.  This allows configurations like

    extlinks = {'quarter': ('https://example.org/quaters/%s',
                          '%s. quarter')}

with ``:quarter:`2``` getting replaced by a link titled `2. quarter`.

The requirement for the caption string is to be either None or contain
exactly one ``%s``.  If neither is the case, then we emit a warning and
fall back to the old behaviour which is concatenating the caption string
with the role content.

We use Pythons %-formatting, so literal ``%`` must be escaped as ``%%``.
Clarify this behaviour for the caption and base URL strings.

The old syntax will be deprecated in 4.x and 5.x and removed in Sphinx
6.0.