doc: promisor: improve acceptFromServer entry

doc: promisor: improve acceptFromServer entry

The entry for the `promisor.acceptFromServer` in
"Documentation/config/promisor.adoc" has a number of issues:

- it's not clear if new remotes and URLs can be created,
- it looks like a big block of text,
- it's not easy to see all the options,
- it's not easy to see which option is the default one,
- for "knownName", it says "advertised by the client" instead of
  "advertised by the server",
- it doesn't refer to the new related `acceptFromServerUrl`
  option.

Let's address all these issues by rewording large parts of it
and using bullet points for the different options.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>