doc: manpage: remove maximum title length

doc: manpage: remove maximum title length

DocBook Stylesheets limit the size of the manpage titles for some
reason.

Even some of the longest git commands have no trouble fitting in 80
character terminals, so it's not clear why we would want to limit titles
to 20 characters, especially when modern terminals are much bigger.

For example:

  --- a/git-credential-cache--daemon.1
  +++ b/git-credential-cache--daemon.1
  @@ -1,4 +1,4 @@
  -GIT-CREDENTIAL-CAC(1)             Git Manual             GIT-CREDENTIAL-CAC(1)
  +GIT-CREDENTIAL-CACHE--DAEMON(1)   Git Manual   GIT-CREDENTIAL-CACHE--DAEMON(1)

   NAME
          git-credential-cache--daemon - Temporarily store user credentials in
  @@ -24,4 +24,4 @@ DESCRIPTION
   GIT
          Part of the git(1) suite

  -Git omitted                       2023-05-02             GIT-CREDENTIAL-CAC(1)
  +Git omitted                       2023-05-02   GIT-CREDENTIAL-CACHE--DAEMON(1)

Moreover, asciidoctor manpage backend doesn't limit the title length, so
we probably want to do the same for docbook backends for consistency.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>