doc: clarify distinction between sign-off and pgp-signing

doc: clarify distinction between sign-off and pgp-signing

The documentation for submission discourages pgp-signing, but demands
a proper sign-off by contributors. However, when skimming the headings,
the wording of the section for sign-off could mistakenly be understood
as concerning pgp-signing. Thus, new contributors could oversee the
necessary sign-off.

This commit improves the wording such that the section about sign-off
cannot be misunderstood as pgp-signing. In addition, the paragraph about
pgp-signing is changed such that it avoids the impression that
pgp-signing could be relevant at later stages of the submission.

Signed-off-by: Cornelius Weig <cornelius.weig@tngtech.com>
Helped-by: Philip Oakley <philipoakley@iee.org>
Helped-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>