docs: document default `User-Agent`

docs: document default `User-Agent`

curl offers a `--user-agent` option for modifying the `User-Agent`
header supplied in its requests.

The man page section for this option explains how to use the
`--user-agent` option, but does not explain which `User-Agent` curl uses
by default.

By default, curl uses curl/VERSION, such as `User-Agent: curl/8.11.0`.

Note that this appears to be different from the libcurl default (no
User-Agent header).

This commit documents the default `User-Agent` in the man page section
for the `--user-agent` option, as well as on the "Art of Scripting"
page.

The `%VERSION` placeholder will be used to insert the current version
as described in the man page generator docs.

Closes #15608